Pros: Lush sound, bass impact, treble extension, packaging
Cons: Clamping force, paint is prone to chipping
Amplifier: Matrix M-stage (Audio-gd SUN v2 HDAM)
DAC: Matrix ASRC Cube DAC, Valab NOS DAC (v2.6)
Other headphones: AKG K702, ATH-ES7, Beyerdynamic DT770, Sennheiser HD555
Great packaging. The HD650 came in a hard box lined with tons of foam padding on the inside. The box is relatively durable and even had metal hinges for longevity. Included a nice 1/4" to 1/8" adaptor for use on smaller plugs. Overall, this is one of the best packaged headphones I've seen.
The Sennheisers fit like traditional circumaural headphones. Because of this, there is a good amount of clamping force. My smallish head does find the Senns comfortable, though my AKG K702 does rank higher. Nevertheless, I've been able to have 4+ hour listening sessions with the HD650 with no discomfort. The earpads and headband foam are easily replaceable.
In terms of build, the headphones are pretty solid. Fit and finish is much better than the HD555 and does not have the same flex. Plastic feels solid and rigid and the headphone cable connection is secure and firm. No creaking on this pair. The paint on the headband is prone to chipping. Repairs for the HD650 by Sennheiser are actually affordable ($60) which is commendable.
I won't make any sweeping claims here, but the HD650 is a very good headphone for its current price. The first thing you'll notice is the darker, lush tone. This darker nature is interpreted by some as a veil, but I do not notice one with my current setup. However, if I substitute a less powerful amp (Matrix CUBE) in place of the Matrix m-stage, I do hear a veil of sorts, though it is not obtrusive.
The bass on this headphones is awesome. Has great impact and PRAT, especially in comparison to the AKG K702. However, it does lack a little of the bass extension that the K702 has. The bass does not intrude on the other frequencies, but does make its presence known. However, the great bass impact also means that the headphones are a bit slower. They won't be able to keep pace with faster rock, although they are acceptable with slower classic rock.
The midbass is where the real magic is. This lends the lush tone that the HD650 are known for. They also give great texture to string and woodwind instruments (something the K702 does not do as well). I find this extremely enjoyable with large classical and most jazz.
The HD650 are often described as dark headphones. That being said, I find the treble to be exceptionally detailed and present. In fact, they extend further than the treble on the AKG K702. This is a double-edged sword as it makes music sound detailed, but also reveals sibilant recordings. Much like the K702, I am able to hear minute details like page turns or the artist's movements. The mids on the HD650 are good, but are not as exceptional as the treble or bass. They have slightly more grain and are not as smooth as the K702. Even so, vocals and piano (which typically cover the midrange) are excellent with this pair.
The soundstage on the HD650 is interesting to say the least. While you do not get the open airy feeling of headphones like the AD700 and K702, you do get excellent, accurate imaging that surpasses most headphones I've heard (even the K702). Not only can the HD650 separate different instruments distinctly from each other, but can also show where they are. On large scale orchestral works it is easy to pick out where every section of the orchestra is.
The Sennheiser HD650 is a great pair of headphones, especially at it's current pricepoint. Its relatively inoffensive nature allows to to excel at many genres (assuming good recording/mastering quality). Good amplification is a must for this pair as it lacks many of its great characteristics without it (imaging, bass imapct, etc). A good source is also key as it does reveal bad sources (not as ruthlessly as the AKG K702). Makes a great complement to airier headphones.