I’ve always wanted to try them both, but the price was more than I wanted to spend. The 650 for $220 is one of the best deals in audio ever. The 600 is probably just as close to perfection but with a slightly brighter tilt on that spectrum. Amazing that Sennheiser made these all those years ago, they are still in production, still so popular, and not entirely outshined by more modern, pricier offerings. I appreciate any headphone that I can listen to for years without needing to EQ it Or even modify it. I imagine the 600 would have been the same for me, but I’ll never know. Maybe I would have even liked it better. Once I put the 650 on my Atom amp... I knew I’d never really desire another high end over the ear headphone. It ruined me.I've listened a lot to the HD6xx. There's no doubt it's a good headphone but for me it had too much "warmth" and bass. I preferred the HD600 and the HD700. Though the HD700 required a pad modification to sound a bit more even in the treble.