I think the reason many find the HD 660 S brighter is due to its stronger overall midrange presence than the HD 650, since the midrange is one of the most sensitive parts of our hearing it doesn’t surprise me more can come off as brighter. The HD 650 does have more treble presence in my experience but the HD 660 S came off as brighter due to a stronger midrange and less bass warmth, despite its highs being more muted than the HD 650. But it has been a while since I heard them side by side, so take what I say with a grain of salt. I did decide to get the HD 650 again, or rather the HD 6XX, which is the same thing. I also plan on getting the HD 660 S again, which I ended up giving to a friend, but I’m waiting for the design refresh to become available here in the US before I do. While I do fall more in the Beyer camp, I realized having one or two of the HD 6 family headphones in my stable is always worthwhile and I always miss them some even though I have headphones I like more.