Pros: punchy bass, strong mids, detailed
Cons: mids are a bit distant sounding, highs are recessed (correctable)
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)
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.