Pros: Tonality, Easier to match with other equipments, smooth sounding, beautiful mids
Cons: For some recordings and with some equipment it can sound bass heavy or too dark, narrow soundstage, colored treble
My reference for reviewing the HD 650 is the AKG K701. I've used the HD650 with various amps and sources including the Nuforce HDP, Headroom Ultra Micro Amp (now discontinued), the Beta 22 (2 channels), Musical Fidelity M1 and the ALO Rex MKII. Sources included Marantz CD5003, Ultra Micro DAC (discontinued), Ultra Desktop DAC (discontinued) Nuforce HDP and Centrance DACPort LX.
I think many reviewers have already talked a great deal about the characteristics of the HD650. Most of them have better experience and probably are more capable of discerning the pros and cons. I only wanted to add a few points regarding classical music listening.
Some listeners shy away from the HD650 when listening to classical for their dark sound and lack of airiness but for my ears they are suitable for the genre. First of all they have a beautiful natural tone for strings and winds, in comparison to the K701 which has an anemic neutral to bright tonality. The HD600 has a similar tonality to the HD650 but IMO the latter is more refined, less grainy and enjoys a better sound stage and imaging. The only drawback is the HD650 need for a neutral to bright system to sound their best. With some set ups it can sound too dark and dull. I heard them at their best with the Beta 22: a very big and tall soundstage with exceptional depth and detail.
When listening to orchestral music on the HD 650 there is more weight and impact. The experience is closer to live performances in comparison to K701. There is also more emotional involvement, using the K701 is like reading the notes, while both HD600 and 650 reveal the mood and the feelings of the piece. In other words, listening to the HD 650 is like being in the concert hall, while the K701 experience is closer to being in the studio.
The HD650 is more forgiving with older recordings. Classical gems like Arrau's recordings of Beethoven's piano sonatas or Oistrakh glorious recordings of Prokofiev and Shostakovitch can't be rendered with revealing headphones like the K701 or the HD800.
It's also much easier to match the HD650 with the right DAC and AMP in comparison to the K701. Actually the HD650 is an excellent performer with high grade equipment and a steal at its current price (around $300 used). For the K701 it's very hard to find the right amp to pair with it, it sounded at their best on the Luxman and MF M1 HPA amps.
The K701 is definitely more airy, with a wider sound stage, but the warm tone of the HD650 provide a solid placement of instruments in space. Sometimes I feel the K701 too thin sounding for complex passages compared to the HD650.
This review was done on mid fi gear involving Cetrance Dacport Lx and ALO RxII.
I did an extensive comparison on the current set up: Flac and Aiff files played by Audirvana plus on a Macbook computer. The macbook is connected by optical cable to Headroom Ultra Desktop DAC. The UDAC is connected to Musical Fidelity M1 headphone amplifier via DiMarzio RCA cables.
On this set up the AKG Q701 is performing very well indeed. The highs are smoother and the sound is more open, with fuller, richer and clearer mids. The imaging and the depth of the soundstage have been immensely improved. In short I feel that the Q701 and the MF M1 amp are very good combo, much better than the HD650 combined with the same amp. Probably the M1 being slightly warmish works better with brighter headphones like the Q701 and the HD800.
The HD650 is certainly more fun to listen to, but the Q701 is more detailed with a wider and bigger sound stage, better imaging and more extended treble. But the tonality of the HD650 is more true to life, where K701 sounds pale in comparison. When listening to the HD650 you feel like living inside the music, while the Q701 is more objective and cold. The mid on the HD650 is rich and has a palpable presence, compared to the thinner mid on the Q701.
Of course these observations are purely subjective, and based on my experiences with different set ups.