The HE-400i is a very neutral sounding headphone, not much emphasis or recession anyway and high frequency extension is better than 99% of headphones. On the other end of the spectrum however, bass extension is linear until 40 or so hertz, then it begins rolling off. I think this is due to the way the earpads are dampened. Typically planars have very linear bass extension, but I don't think its that problematic, especially considering most open-dynamic headphones are much worse in this regard. One thing you would expect from a planar is a smaller soundstage, and the HE-400i doesn't fail to meet expectations there. While the soundstage doesn't feel clausterphobic, it lacks a sense of air. Songs that have big soundstage are conveyed decently, but ordinary songs are pretty stereo sounding in a very left and right way. I'm a big fan of huge soundstage, so the HE-400i doesn't exactly excite me with how it portrays the soundspace, but the imaging and clarity are very good, as well as depth. Timbre, texture, realism, etc. are some of the best aspects of the HE-400i, combined with the quick decay the sound of the HE-400i is just really fast and responsive, never gets congested in genres such as dubstep or metal.

I think they're quite good, but not for everyone. I bought them because I wanted a more open and comfortable counterpart to my JVC FX850 IEMs. Based on technically merit alone, they're not much superior to the JVCs (well, FR is much more neutral) and comfort-wise I have a very large head so the clamping force makes them uncomfortable for me. Many others who I've had wear them say they're remarkably comfortable, though. I think I'm going to sell them and try the HD600, AD2000, DT150 or K612. 

The Sennheiser HD280 that I'm reviewing do not belong to me, they're presumably a couple years old (I didn't ask) and in good condition aside from a couple scratches on the earcups. I was curious to how they sounded, especially in comparison to the HA-RX900/700, ATH-M35 and M-Audio Q40 (all of which I don't presently have but definitely remember quite well). I do have Sony XB90EX, Fostex T50rp II (modded to be flat aside from a spike at around 8 - 10 khz) and MDR MA900. All very different headphones, but yeah on to the review.
 
Build Quality: 4/5
These are solid feeling headphones, the cable seems nice but I suspect it's on the cheaper side due to the fact these insensitive headphones have an audible whining similar to the Q40's stock cable, which I know for a fact is awfully cheap. I like the way the earcups swivel, all the moveable parts seem durable. (the 4/5 is because the cable quality, most certainly).

Comfort: 3/5
These are light headphones with poor weight distribution and strong clamp. I get a mild headache-like feeling from wearing them for about a minute, and I'm used to ridiculous clamps (hint hint, Q40). The earpads are deep, however they're not well padded and don't feel that great with the strong clamp, but I'm not very sensitive to this kind of thing so it's fine for me. I know a lot of people would find these dreadful to wear, but I suspect the clamp helps with the isolation which is decent. At a lesser price, both the RX700/900 and M35 are much more comfortable, though the RX700/900 has a somewhat awkward fit and the M35 has weird pressure points, neither are as uncomfortable as the HD280.

Price-to-Performance Soundwise:
The HD280 is a mixed bag of nuts, for sure. I've been listening to them for a few hours and have adjusted to their signature pretty well (all my headphones have pretty different signatures, so this is no problem for me). Plugged into my Sansa clip+ these sound almost exactly like an RX300/500. (15$ headphones with very mediocre sounds, but acceptable at their price). The cable noise isn't at all present with the Clip+, but the headphones sound thin and slightly less refined in the lower frequencies with an oddly out of place and thumpy subbass which seems to come from nowhere. I think it's more accurate to say they're an inbetween of the RX500 and 700, leaning closer towards the 500.

My source and amp is the Schiit stack Magni/Modi, and with it the 280 sounds a tad better, but the cable creates a background hiss which is quite annoying. For a proclaimed 'monitor' headphone, the subbass is a bit too pronounced and a tad uncontrolled, but not as bad as you'd expect.The bass lacks texture big time, but it gives the headphone an illusion of a fuller sound. The treble is rolled off quite a bit after 10khz but before that there's a noticeable peak that makes it sound bright/harsh. The upper midrange is very recessed too, making vocals sound muffled and murky in comparison to the lowest midrange and upper bass. The soundstage is oddly wide, but it sounds almost as though it's artificial, like encased by a plasticky sounding barrier. Soundstage lacks depth and height, being purely left to right and wide. Pleasant lack of midbass hump, which the M35 have but not extremely so --- however the subbass emphasis is more distracting in a way. The overall clarity and imaging of this headphone is mediocre, even if the soundstage is big I have trouble picking out details. The M35 has a smaller soundstage, but it outdoes the HD280 easily with detail delivery and imaging. Comparison to my MA900 isn't necessary, as the difference in quality is just that large. While the HD280 doesn't sound awful, it should NOT be used in any kind of studio. It's not flat by any means, recessed upper mids, big treble-roll off and some subbass emphasis make it a poorly coloured and undetailed headphone. Both the M35 and RX700/900 sound better, objectively and subjectively. RX700 is actually the superior of all three, even though it's priced at a mere 30 - 40$ about half the price of the HD280 and the M35.

Direct comparison with my T50rp II (modded) (a large amount of fiberglass, a little cotton and clay loaded baffle plate with 3mm ports on the vents. Also, I use Alpha Pads with it and I've covered the 'reflex dot' or whatever it's called (pardon me if I'm wrong)). Ten songs:

Bassotronics: Bass, I Love You
HD280: Bass is audible, doesn't go very deep. 
T50rp II: Bass is more subtle, can't really tell how deep it goes because it's hard to hear over the rest of the song.
 
Bonobo: Jets
HD280: I kind of like the way it reproduces the female vocales in this, but the imaging gets a little blurry when the background instruments play alongside it. It's hard to hear the details in the background, and the harp sounds a little dull. The synths sound pretty good, though --- the subbass has a bit of authority behind them that doesn't blurr with their midrange frequencies, probably due to lack of midbass emphasis.
T50rp II: A little bright sounding in comparison, bass isn't too different but it lacks a sense of bloat. The synths in this sound similar to the HD280, but lack the subbass boost and sound a lot cleaner as a result. Details are very easily picked out, though the soundstage isn't that big (it's at least bigger than the 280's) everything is there and easily defined albeit a bit on the bright side.


The Dirty Heads: Stand Tall
HD280: The overall sound is a bit dull, but wide. Guitars have a rounded quality, almost as if they're a beginner guitar you got at a pawnstore for a 20$. Imaging isn't that bad in this song, but the background singers can sound really blurry. Treble sounds etchy, almost like the song is of a really low bitrate (such as an 240p youtube version) and the drums sound off in this song. Vocals sound alright though.
T50rp II: Immediately I noticed the huge difference with the bass guitar, much more texture and it blurs a lot less. Vocals sound good, less thick than the HD280 and a bit more realistic. Drums are excellent, sharp and natural sounding -- I can almost hear the acoustics of the room it was recorded in in the way the drums sound. Background singers don't blend in with the other instruments, so that's good. 

Excision: 8 Bit Superhero (Eptic remix)
HD280: I can't say the HD280s belong here. Soundstage is really small and the general sound is very bland, the only thing the HD280 does right here is the subbass. Everything sounds muffled.
T50rp II: Switching to the T50rp II I immediately notice all the sounds that sort of blended together. You can hear the electronic sound and all it's texture with the T50rp II, but it sounds a little thin. The bass is less quantiful but extension is better and it also transitions to the midrange a lot less strangely than the 280.

Flux Pavilion: Hold Me Close
HD280: I like the way the HD280 sounds here. Soundstage is about right, good sense of wideness without much blurring into the middle like it usually does with other songs. The subbass quantity goes well with this song, but it sounds kind of funky considering the hollowness of the rest of the spectrum. I notice that the HD280 seems to have pretty bad decay, this song makes it easy to tell.
T50rp II: At first the sound is a tad on the brightside, but the midrange sounds excellent with the synths here. Vocal samples are really smooth and realistic, well as realistic as they can be in this kind of song. Bass is good as always, a little shy in quantity but good nonetheless. Overall clarity is top-notch, but the treble sounds a little too smooth, but it's probably just the way the song is.

Jack Johnson: Crying Shame
HD280: The 280 trashes the vocals here. This kind of sound just makes me feel like I'm listening to 20$ headphones, and not very good ones. Treble is a bit grainy, also is kind of similar to sifting sand. The drums sound really dull, like someone covered the heads with cloth or something. Bass guitar sounds relatively good, has plenty of authority and sounded and sounded well controlled, still not much texture though.
T50rp II: Wow, just wow. This isn't the same song, ist it? Vocals are super smooth, though you can easily pick out the artificial reverb-ish effect added to it. Drums are tad bright, but they have a nice clean sound with good impact. The bass guitar is a lot less pronounced, but it's got excellent tonality.

La Roux: Tigerlily
HD280: The vocals sound alright, but everything else seems to lack dynamics and sounds really flat. The various sounds kind of blend together and overall this song is muffled sounding. The bass has a nice impact and visceral quality in this song.
T50rp II: The vocals sound really close to being thin, but it's more like they're very neutral sounding. They sound a tad further back then the HD280s, which seemed to place them really close to you. The song itself has an interesting crispness, bass is a lot subtler and lacks impact but when it hits it makes you feel like it's really low. 

Pink Floyd: When the Tigers Broke Free
HD280: The 280 portrays the large sound of Pink Floyd quite well in this song. Vocals sound somewhat raspy/nasally(?), but don't sound muffled. The subbass emphasis makes the drums sound huge and powerful in this, but the sense of space is ruined by the higher drums sounding so unnatural. Things get a tad congested towards the end of the song, but not horribly so.
T50rp II: You can hear the quieter details more easily here, the main vocals are a touch too bright for my likings. The big drum in the background doesn't sound as big as it does on the 280 but you can hear the springiness it has after impact, the background singers are also more easily heard and you can almost pick out different voices. I also noticed a couple instruments in the background than I did with the 280, and the sense of space here is huge for a closed headphone.

Kanako Itou: Space Engineers / 宇宙エンジニア(uchuu engineers) (orchestral version)
HD280: Kanakou Itou's voice sounds a bit muffled here, as well as the instruments but not as bad. Compared to other songs, the soundstage and dynamics are really squashed sounding. I could try, but I don't think I could ever enjoy this song through these headphones.
T50rp II: Instruments sound a tad tinny but the sense of space is huge and the stringed instruments have a crazy amount of texture. Kanako Itou's voice is rendered really well, not as realistically as the MA900 (woops, didn't mean to throw that in but it is my standard for naturalness) can portray it but good nonetheless. The instruments in this have a really nice attack and the dynamics are compared very well in comparison to the HD280. This song is hard to get to sound right, neither headphone sounded natural here.

Michael Jackson: Leave Me Alone
HD280: The 280s lack of treble is pretty clear on this song, it sounds a lot like an old car stereo here. There's a decent sense of space, not much detail blurring. Bass is probably the best the 280 does in this song, probably because there's not a notable amount of subbass and the 280 is pretty modest with it's midbass. At this point, I can't really say much that hasn't already said about the 280 in all the other songs. 
T50rp II: I expected the T50rp II to be really bright here, but surprisingly it was only a little bright. Michael's voice seems about right, the soundstage in this song is pretty small so it sounds a little closed in. Something about the T50rp II bass has a subtle impact and speed which makes this song oddly engaging. Like the 280, not much else to say at this point.

Summary of HD280 vs T50rp II (modded) comparison:
My T50rp II is a good bit brigther than the 280 and lacks the severe rolloff the 280 has, making is a little harder on the ears. The 280 is extremely unresolving in comparison, and despite not being bright and a little bass oriented it still sounds like a very dull and lacking headphone, especially at the price of 80$. A modded T20rp II would be a much, much better value than an HD280. I honestly don't think I could ever seriously recommend the HD280 to anyone, it's most positively feature is it's build and that can only get you so far.. the JVC HA-RX700 is a far better alternative at a far lower price, for those who're concerned. For those deciding between the HD280, the M35 and the V6/7506 I'd very seriously recommend either of the two latter, even without hearing the V6 I know it's at least slightly more detailed than the M35 which ultimately makes it very clear in comparison the the HD280. I'm sorry owners of the HD280 who enjoy it, it's a very poor headphone at it's pricepoint and I wouldn't pay even 30$ for it. I didn't really mention the XB90EX much, but despite it's elevated sub, mid and upper bass (by 6 decibels) it still manages to deliver more clarity than the HD280, costing only 20$ more and being a bass oriented IEM I think that's just ridiculous. ---- disclaimer, I don't believe in physical burn-in much, but mental burn-in is a big factor so I made sure to give the HD280 a good few hours of listening time before deciding to review it. At first listen I literally thought it was identical sounding the the RX300, with added subbass. After a few hours it got closer to the RX700, but that's a generous thing to say. I'm dissappointed, this is the 2nd sennheiser product I've tried and I don't think I can appreciate a company that places no value in it's lower priced products and don't think I'm interesting in buying any of their other headphones, --- which generally have a reputation for being overpriced anyway.






 

I got these off a friend for 40$, they go new for about 60 - 70$. The driver appears to be of Audio Technica origin and the fact it's 53 mm, which makes my guess a lot more likely. Sound-wise it's a bit artificial but expansive sounding for a closed headphone, changing the pads helps make it sound less artificial. Comfort is alright, they're not as heavy as my T50rp II and are a little heavier than my ATH-M35s and they're kind of large but adjust just enough to fit my head, which is subjectively on the larger side. They're very easy to drive with my Schiit Magni, and I notice when I turn the knob past halfway there's a very large amount of noise so I'm assuming that either these headphones have a high noisefloor or they're overly sensitive. They're really loud at 1/3 of the Magni and they've got a flat if not basslight sound which isn't bad but leaves you wanting a bit, if you're me. The cable is coiled like an old fashion telephone and is a bit short and obnoxious, but who really cares at this price. The construction is seemingly solid and some parts are even made of metal. I'd recommend them for anyone looking for a closed headphone under 100$.

I bought this headphone twice, because I like it a lot. They're good for the money, not perfect but definitely great. First, I'll talk about build quality and comfort, then efficiency and sound. (and also a bonus category of how moddable they are)
 
TL;DR review:
9.5/10 overall. Sound/value easily 10/10. Comfort 9/10. Build quality 8/10. Does not need amp, benefits from one. They're easy to mod for beginner DIYers.
 
1. Build Quality: 4.5/5
 
I'd say the build is the only part that shows they're a cheap headphone, don't get me wrong -- they're solidly built but they creak a lot on your head. They aren't prone to scratches or chips, which is probably because they use a decent plastic. The cable is nice, very thick and lengthy w/ no audible microphonics. The headband is interesting, as it's not a traditional headband. I'd say this is similar or possibly inspired by Audio Technica's wing design, but is simply a padded cloth-like band that bends to your head shape and is held up by a more solid plastic arch that's above it (the cable goes through this, since it's single-entry.) The ear-pads are nice, too nice for the price. They're thick enough, and they're really smooth and soft. Often mistaken as leather to those who aren't very familiar with leather (though, they're far less breathable than leather -- better for bass, worse for heat insulation.)  These headphones aren't tanks, but are solid enough. I use them for portable use (not in-between locations, but at school and at home) and I find them pretty easy to carry around. They're not very heavy either, -- for their size. 
 
2. Comfort. 4/5
 
The ear-pads are a godsend, very good pleather. At first the pleather is a bit too firm so I continue to use my first pair of RX900's pads on my second pair (am keeping the new earpads to use in the future). After they break in they're extremely soft and cushiony, not as thick as say, the Sony XB-500s but adequate and probably finer pleather. The cups are angled so they fit oddly, I usually mess with the pads orientation on the cup and the headphones orientation on my head but that's not very bothersome to me, just kind of weird. For smaller heads these would be perfect, despite being huge they're more suitable for those with a smaller head since the cups push inwards -- on my head the cups are forced outward so it's got an unnatural clamp I mostly don't notice, but oh well. The headband, as I mentioned before is a padded cloth-like band that is supported by a plastic arch. The headband is also a godsend, initially it's firm like the earpads but breaks in nicely. I think the headband is better than any regular headband type no matter how much cushion you use on it. A lot of people mention the fact these headphones get hot fast, and this isn't incorrect. Despite being semi-open these hold in heat more than the normal closed-headphones. I suspect this is because of how fine the pleather is, it's really not a problem for me most of the time but I guess you can say these aren't just warm sounding . Overall though, the comfort is enough for me. People with smaller heads would probably find these headphones much more comfortable, and that says a lot because they're already quite nice.
 
3. Efficiency 3.5/5
 
Not much to say here, they are what they say they are. 64 ohms 106 db sensitivity. They can be driven by iPods, but anything less than 50 milliwatts per channel won't work for these. They definitely tighten up with an amp, not dramatically. I personally think they sound weird with an iPod, but fantastic out of a DAC w/ an amp. They can handle 1500mw max.
 
4. Sound 5/5
 
Don't be mislead by the 5/5, they sound spectacular at their price but don't think that means they're without flaws. They have a very large soundstage for a closed, even semi-closed headphone (I say this because they isolate and don't leak much despite their open-like soundstage) The lows of the RX900 are great, they extend well and have decent control --- can be improved with modding (more on that later) and have great punch at all volumes, it's a weird kind of punch but it's very realistic in a way. I'd say they have more punch than M-Audio Q40s but unlike the Q40s the subbass doesn't have punch. The lows have a natural presentation, does bass guitars and drums justice --- lacks a bit of clarity in the upper lows, weirdly but not isn't really noticeable to people who don't focus on bass as much as me. The mids are both the strong point and also the weakness of these headphones -- they're wonderful. The lower mids are a little less refined than the other portion of the midrange, but not much. Voices and guitars come out really well due to the mids, but, BUT, the mids aren't very forward. It's weird, it's not like they're quiet but it feels like they're further away and in some songs this is annoying, mostly sounds with a lot going on at once. The highs, are definitely the worst things about the RX900s. They are detailed enough, just slightly muffled and recessed. There's a weird dip in the upper midrange and after 10 khz things start to go down fast, (this is due to a felt ring placed in front of the driver, and it's easily removable).  Overall, I'd say they sound good out of box and after burn in,  and if you don't believe in burn in I still strongly suggest you let yourself get used to their sound because it grows on you, a lot.
 
5. Bonus - modding.
 
These headphones are one of the most easily modified headphones I know of. The cups are large so there's a lot of space to work with and there's even a few mods that don't require you to open them up -- like stuffing the earpads so the driver angles more directly at your ear (brings out the midrange), or simply replacing the pads to improve comfort(?) or bring out or tame some frequencies. On the note of bringing out some frequencies, there's a thick felt ring placed over the driver under the earpads, I find it mandatory to take it out -- it kills the highs. As for opening them up, there's a lot of things you can do, mostly damping. These headphones (stock) have an artificial hue to their sound, a little damping can really take them to the next level. TopPop has an excellent basic modding tutorial http://www.head-fi.org/t/381303/jvc-ha-rx900-modifications-a-picture-tutorial-56k-forget-about-it and I'm sure you can alter materials and ways of damping to your liking. I myself decided to just take my RX900s and completely opened them up, greatly reducing the need to improve the sound via damping. Results: They have a larger soundstage, far more natural highs, (and everything else) and possibly more punchy and present lows. The mids are still distant, but without any weird resonance or colorations. I think I'm the only one who has tried completely opening them up, I can't say for sure if there's any abnormalities in the frequency response, but it sounds much more refined now so I wouldn't think there is.

The M-Audio Q40 is the first high quality headphone I've owned, despite this I can still be honest and accurate about the qualities of this 'phone without overselling it.

Let's start with the build quality and design. The plastic that this headphone is mostly constructed out of is rigorous and it'd take quite a bit of effort to break and in no way feels cheap, brittle or flimsy. The headband is malleable to a degree, but I'd be careful and not overestimate it's flexibility or test it's limits because it's definitely capable of being bent or stretched to any head shape, --- which I've actually gotten it to do for me, despite my larger than average head. The headphone cups are good too, but honestly the additional pseudo-grills they added onto it are loose and can creek and cling sometimes when you're adjusting the headphone on your head or touch them, but for the most part aren't really noticeable or problematic in anyway. Now, here's what really urks me about these headphones; on BOTH sides of this headphone about 3.5 inchs of wire is exposed and for me I can't help but worry that one day this'll be the thing that does these headphones in. Other than that, construction is top-notch and I wouldn't expect it to break unreasonably.

Comfort is mediocre in these but not as terrible as some would like to state. Once you stretch them out a bit they actually can be worn quite easily for a few hours at a time. The pads are pretty thin and look/feel like a cheaper type of pleather but are still pleasantly soft despite being thin and aren't too bad. I'll honestly say the first time I put these headphones on I couldn't wear them for more than a few minutes without having a discomfort-caused headache, but at this point I'm accustomed to a strong clamp and they've loosened up a bit so I can't really complain.
 
Now on to my favorite part: The sound of these headphones.
For this part of the review I'll explain how this headphones fares in certain situations and what it's like.
 
So without any equalizing, the Q40 is a slightly bass-emphasized headphone with good, clean mids that are a bit less than what you'd say it 'forward' sounding, so vocals on some tracks are a bit softer sounding than the instruments but still quite clear and good sounding -- to elaborate on that, they sound very natural to me and are very pleasant. (which is unusual for bassy headphones) -- the highs are good, but there's a small 10k spike and a pretty big roll-off thereafter. I personally could do without the 10k spike (and usually do when I apply an EQ) but the rolloff for me is a big plus, because I can't stand fatiguing treble and my ears are unusually sensitive to higher frequencies. The lows are probably what most people are interested in, so I'll get right into detail with those. These headphones aren't all about mid-bass, which is probably the bass 'bassheads' per say, would be looking for. I am a self-acclaimed basshead and I know a lot of bassheads originate from a listening experience with the Sony XB-500 or Dre Beats, (I know I did --- XB-500s, not Beats, I don't even need to elaborate the silliness of the latter) which is very mid-bass heavy. The Q40s have really good sub bass, a type of bass most headphones don't even touch. Every frequency from 20 - 300 hz is at least at and/or above neutral and 50 - 100 hz is the most elevated frequencies (100 hz being the peak). 50 hz going towards 20 hz is like a slope, but it's still elevated, audible and present. Now, going past the quantities aspect, I'll start off by saying the Q40s bass is tight, very fast and well-textured. This is highly evident when listening to metal with fast, low, hard-hitting drums and these headphones really don't miss a beat. Despite the lows being highly present in this 'phone, they're very proper and not intrusive to the mids or highs and don't make a big mess of everything. Quality over quantity, a lot of people would say and prefer --- HOWEVER, the bass quantity in these headphones to me is unrivaled by most headphones and just a bit below XB-500 level, DISCLAIMER; with a bit more power than an onboard sound card or normal iPod offers they are really on-par with the XB500 (un-equalized) and with a bit of bass boost can easily rival the XB-500, not that it's necessary cause the sound of this headphone as a whole presents it's self in such a lovely fashion that much alteration isn't necessary to be satisfied. 

All in all, I'd sum the sound to say they're a warm, clear, well-defined sound headphone with excellently present sub-bass and decent mid-bass. The midrange is perfect, for me, and if you get rid of the 10k spike there isn't a thing wrong with the sound of these headphones, unless you really like far-extended highs and overly bright headphones, then I guess these would feel a bit lacking in that aspect. For casual listening at home and in quieter place, there's no headphone I'd choose over these. For portable headphones and in loud places, these do work for portable headphones, but with the exposed wires and need to turn them up really loud I really find it more on the inconvenient and worrisome side.

*update, over a year later*

I've long since sold my original Q40 and bought a new one. I modded the new one with a decent amount of fiberglass and cotton, as well as replacing the stock earpads with 840 pads. Stock earpads sound best, but comfort is important too. The clamp on these headphones is a bit much for extended listening. I usually don't listen to these without equalizing down 100 - 200 hz by about 3 decibels. When equalized they're a really good sounding headphone for 100$ (what I paid the second time). They have a smallish soundstage, but when equalized like I do they have a very nice, textured sound that makes them a very fun listen, preferable sometimes to my MA900 and T50rp II. Build quality is questionable, I know the wiring is ****ty but I don't think the headphones will fall apart anytime soon. Stock cable is mediocre, it wines when plugged into my Magni. I'd say you can't go wrong at 100$, but as you get closer to 200$ there's many more options, most of which are objectively better -- though, not as much competition for a good basshead 'phone. I'd say these are clear sounding without equalization, the upper bass makes them seem muddy, though they aren't. After extensive research, I re-purchased these instead of the DT770 and I'm glad I did, aside from the comfort issues.