I've been using a later pair of HD650's for a while now and I have to say they are the most comfortable headphones I've ever had and as for sound quality they are very good, from deep, deep bass right up to high treble. I've no plans to change them as they do everything I want.
Turntable
Toshiba SR-370/Mission 774/Van Damme cable with MS Starline plugs/Ortofon Quintet Black
CD
Cambridge CXC transport/modified Musical Fidelity X-DAC/modified Musical Fidelity X-10 V3 tube buffer
Network Player
Cambridge NP30
Amplifier
Denon HA-500 head amp/Nakamichi CA-5E pre-amp/Chinese passive RVC/Proton AA-1150 DMC power amp
Speakers
Mordaunt Short Signifer on original factory stands
Cables
Mogami with Rean Neutrik plugs/NVA LS5
Headphones
Sennheiser HD600/Sennheiser HD650/Koss Pro4 AA