Oh yeah, the HD660S are definitely underrated. I’ve used the 600, 650 and now 660s, and I prefer the last one, although The 600 are my “first love” headphones lol (I’d buy them again some day). The HD600 are great (specially they’re sound signature), but sometimes too clastrophobic to me, the HD6XX/650 on an another hand are a bit too warm for my taste, their soundstage isn’t that better than the HD600. And here are the HD660S which are faster, energetic, having a large enough soundstage for my taste, and their detail retrieval is just amazing. They’re hated for not being smooth, and yeah I see that on some Jazz/Pop songs, but other than that they’re much better for my taste.
The HD6XX is a very chill/smooth sounding headphone, and you’re coming from the HD 599 which was a pretty much oppossite lol