Pros: full and intimate mids, soft treble, scales excellently, detailed almost every part is replaceable, easy to run balanced, bass warmth
Cons: slightly soft sounding, mid forward amps can cause brashness, some grain in the treble, bass rolls off early in the sub bass, clamp, ear cup depth,
I bought the HD650 after owning a number of equally good headphones, including the Beyerdynamic DT880 Pro, DT990 Pro, AKG K702 65th Anniversary, and AKG K612 Pro. I'd have to say that while the HD650 has many competitors nowadays that it didn't have when it debuted in 2003; especially lower than its MSRP of USD 500, it nonetheless holds its own and still offers qualities that many of them do not offer.
Build-wise, the HD650 is a timeless and no-frills modular design of mostly plastic and some metal. The outside headband, the bales and the ear cups are plastic, while the inner headband is spring steel and the outside earcup grill is some sort of metal mesh. The headphone can be disassembled by hand to its component parts and basically every part can be purchased from Sennheiser and replaced. That's something you don't see on many headphones today. The dual sided cable is detachable and has two pins for each side, making the headphone easy to run balanced. Just unplug the included cable and plug a balanced one in. The stock cable has decent enough shielding, is dual channeled, is about 10 feet long, and terminates to a 6.35 mm stereo plug with an excellent 6.35 to 3.5 mm adaptor cable that will not put strain on the 3.5 mm output jack of your source device. The cable doesn't tangle easily and doesn't retain kinks. I would have liked the y-splitter to have been a bit lower but that is all I have to complain about. I have not used a Venus Audio or Cardas Canare cable on the HD650, so I can neither confirm nor deny whether they are worth the purchase. My biggest complaint is that replacement ear pads are 60 dollars from Sennheiser, making them very expensive considering they have a tendency to flatten out after a couple of years. There are third party Chinese made ones on eBay for half the price though.
Comfort-wise, I didn't originally like the HD650. I found the spring steel headband, while extremely durable, made it a vice grip on my head, causing a headache. Another side effect of this clamp was that the pressure caused the velour ear pads to prematurely collapse, leading to my ears pressing against the foam driver disks, making them red hot and sore. I find that the HD650 doesn't have deep enough ear pads, and the arey fairly narrow, causing the edges of the pads to rest on the backs of my ears. I found that stretching the headband out (by taking the ear pads off and placing the HD650 over a speaker cabinet overnight) lessened the clamp, and rotating the cup pivots to put more clamp on my temples made the ear pads keep their shape, and now my ears are no longer pressing against the drivers. I find this to be a lot of fiddle work just to make a headphone comfortable enough. Removing the foam driver covers does give a little more depth, though it makes the sound a little brighter. This might be desirable for some people though.
Personally I would have liked Sennheiser to have stuffed the HD650 ear pads with a denser material like gel or double memory foam to keep the shape for longer periods of time.
The hand band is a nylon-type material and has plenty of padding. There is an indentation in the middle of the pad so you can rest the headphone on a stand and not dent the padding, and for people with a Sagittal rest in the middle of their heads, this can provide a little more comfort as the headband isn't hanging up on the ridge and causing aches from hot spots.
As amplification goes, the Sennheiser HD650 is a headphone like prefers a powerful yet somewhat lush sounding amplifier or tubes on an amplifier. Though it is 300 ohms, the HD650 is quite a sensitive headphone, which means one will not have to crank up the gain on an amplifier to get a satisfying listening level, and will not hear as much amplifier strain. I was able to listen to the HD650 at perfectly satisfying listening levels from the headphone jack on my iPhone 5, and it also sounded good on the FiiO E07K portable amp and DAC I owned. The HD650 is more performance hungry than gain hungry. The better the amp to feed it power, the better it will sound. The HD650 isn't as system-picky as AKG headphones like the K702 or K712 Pro, so it is easier to attain a better sounding synergy with. The HD650 DOES prefer an amp that has a powerful sound to it, so solid state or hybrid amps are more preferable. All-tube amps such as the Woo Audio WA6, WA7 Fireflies and WA6-SE are supposed to sound excellent with the HD650 due to their transparency and power while still being refined. Higher end solid state amps such as the Violectric HPA-V200, Burson Audio Soloist and Conductor, and Meridian Audio Prime are also said to be great with the HD650.
I personally use my Schiit Audio Lyr 2, which is a powerful sounding yet thick and refined hybrid headphone amplifier with the HD650s. It still doesn't make them bass cannons or treble monsters, as the HD650 will tend to keep its softer sound regardless, but on low gain it has plenty of power and refinement. On high gain, however, the Lyr 2 becomes a bit too gain happy and aggressive with its stock tubes on the HD650, making the midrange and lower treble too forward and splashy sounding. Low gain is the better setting and is more controlled sounding with the gain-sensitive HD650s.
Now for sound:
The HD650 was and still remains unique, as unlike most Germanic headphones, and more similar to Japanese headphones, its sound is definitely tuned by the ear. And what I mean by this is that rather than being tuned to dig from the lowest to highest frequency, HD650's sound is slightly n-shaped with an intimate midrange, a polite treble and warm bass.
Treble-wise, the HD650 has very good extension with some elevation in the very top end. I don't consider it to have rolled off treble like newer Sony headphones have, though it's not a highly dynamic and front stage treble like on some Beyerdynamic headphones. It's present, yet subtle. The lack of a ton of lower to mid treble DOES make the HD650 a little less than perfect for orchestral music due to less airiness, but it still has enough texture for acoustic music. That said, the HD650 will still show sibilances and some grain with poorly recorded music. Extremely sibilant tracks, especially on lower powered systems that may strain somewhat, will still have some harshness, even on the HD650, and the slight graininess to the lower treble can add to this. But it's infrequent enough that I don't consider this to be a deal breaker in any way.
Mids are the true forté for the HD650. The mids are fairly intimate and forward from the lower midrange to the upper midrange. After listening to the HD650s, many other headphones will often sound "hollow" or "scooped", with some withdrawal or midrange recession in comparison. Female and male singers alike sound equally strong, and instruments have a natural timbre to them. People who love a front and center midrange without the added brightness of the treble will love the HD650. There is little to fault. As I mentioned in the amplification section, though, watch out for amplifiers that are overly forward sounding, as they can make the mid-forward HD650 harsh, brash or blarey. Headphone amplifiers and tubes with a neutral to somewhat soft midrange work best with the HD650.
Bass is an interesting point for the HD650s=, and along with treble, is responsible for the HD650's highly enjoyable midrange. Upper to mid bass notes are forward, textured and extremely present on the HD650. It does not have that overly "groovy" sounding bass like the AKG K240 MKII/Studio due to a highly boosted upper bass, but its bass is more upper and mid-focused than say, a Beyerdynamic DT990. This creates the more filled-in sound of the HD650's mid range. Wind instruments and bass guitars are excellent on the HD650. The mid bass has some bloom to it, and while not the tightest, lends well to drums, and synthesized mid bass with house, trance and dance music. The sub bass, however, is the HD650's weakest point in its low frequencies, as it rolls off after the lower mids. This makes it not the best headphone for genres that need a more filled in or neutral low end such as hip-hop and drum and bass, or genres with powerful drumming such as Japanese Taiko drums. Due to the lack of low end bass, but extra upper to mid bass, the HD650 has a somewhat soft or liquid sound to it. The extra upper bass gives the HD650's bass timbre a more chesty or "wooden" sound that lends well to African, Arabian and South Asian genres of music.
Soundstage, while being good with depth, lacks the layering and precise imaging of other headphones. This gives the HD650 that characteristic "3-blob" dimension to its imaging. You hear music from outwards, up and down, but instrument placing and layering is not as distinct. I would say that for symphonic and orchestral music this is probably the biggest weakness of the HD650.
There are a number of alternatives to the HD650 one might wish to consider instead. The obvious one is its older but still produced sibling, the HD600. The two are basically the same headphone design, with the finish of the plastic being different color schemes, the HD600 being a little less expensive, different headband padding, and the HD600 cable being thinner and terminating to a 3.5 mm connector. Both have a similar, mid-prominent lushness to them, but the HD600 gives up some bass energy and trades it for a bit more treble, which gives it a more acoustic and orchestral-friendly sound somewhat closer to neutral. The two from what I heard are similar enough that owning both isn't worthwhile unless collecting is an endeavor.
The AKG K702, Q701, and K701 are often compared to the HD650, but they are lean and bright enough that they should be compared more to the HD600 than to the HD650. I owned a Q701, and it is a completely different sounding headphone, with deeper but harder ear pads, lesser grip but a bumpy headband, a zazzy upper midrange, bright lower to mid treble, tight and neutral though dry bass and a soundstage with outstanding airiness, layering, texture, dimension and imaging that made the headphone excellent for jazz, orchestral and symphony, and acoustic music. Though the strong upper midrange made the Q701 a bit too aggressive at anything above moderate listening volumes for trumpets and saxophones, and women singers who sing at high and loud octaves (e.g. Adele, Rebecca Ferguson).
A fairer comparison would be the newer K712 Pro, or the K702 65th Anniversary Edition, as their sound is more comparable to the HD650. I have compared the HD650 to my brand new AKG K7XX Massdrop Edition that is tuned to sound similar to both the K702 Annie and the K712. Compared to the HD650, the K7XX is more comfortable and less tight fitting. The K7XX has more treble and more sub bass, while being a little more withdrawn in the mids than the HD650. The K7XX like most AKGs is highly source-dependent. It will sound like garbage on garbage recordings, and it will sound sibilant on poorly matched systems, unlike the more source-forgiving HD650. I will be writing a more detailed comparison review of the K7XX and the HD650, so more comparisons will be found there.
The closest Beyerdynamic for comparison is the DT990, as the DT880 is flatter and more comparable to the HD600. The DT990 Pro is less expensive than the HD650, and has a prominent u-shape to its sound, with very dynamic and bright treble, deeper and more rumbling sub bass and mid bass, a wider and more open soundstage, and a more scooped midrange. The HD650 is definitely the softer and more laid back of the two, and the DT990 the funner, more exciting sound. Both make excellent complements to one another for different listening moods. The DT990 Premium models are a little more neutral, with the DT990 Premium 600 ohm having the smoothest treble of the 990 models, and being the closest competitor to the HD650 due to its refinement.
In the planar range, Hifiman's HE400i is priced at the same 500 dollar mark as the HD650. From what I have read, the HE400i has a strong clamping force, is somewhat heavier, has deeper extending and tighter bass, and the treble is more energetic.
Another option to consider is the recently released Philips Fidelio X2, which according to reviews from owners, has a tighter and more present low end than the HD650, a more neutral treble, and a wider soundstage.
While the HD650 at times can be soft sounding and isn't the most comfortable for long listening sessions, it nonetheless will remain a staple headphone in my fleet. I'm interested in seeing what upgrades to DACs and better tubes will do to improve refinement.