I have had the HD600 a few times and as @iFi audio stated above, it is really a huge bang for the buck. Even though the HD660S2 is at a higher price ($600), it really is also a huge bang for the buck in todays HP market. I would consider the HD660S2 an HD600 with a little added musicality speckled into the sound (a little of HD650 flavor added to it).I've never been quite happy with my headphones, my short audition of the HD600 sounded glary, and in owning the HD6XX for 3 years I only managed to use them 20 times or so, too fuzzy sounding. Everything else that came my way I ended selling either due to needing excessive EQ to the point where the stock tuning wasn't worth my time, or they were just too big/heavy for day to day use.
But I've come full circle, got marble HD600s and love them. Now I'm selling my Aeon Noires and T1.2. Got a mod kid on the way with ZMF pads and I'm quite content with the $200 they cost in comparison to the other stuff I've had.
I still need a good closed back set, I've got DT770s as backup in a pinch, but those I only like for gaming. So I might consider the D9200 at some point. The Aeons are good, but them requiring too much power means they're useless for most of my scenarios. And while good at everything, they just don't excel at anything in particular. And for gaming their imaging is unreliable at best.
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