I personally did find the mids on the HD 660S better than the HD 660S2. I felt the S2 pushed the upper mids a bit too back for an HD 6xx series headphone. While this works with say Beyers due to their more dynamic nature, it didn't for the S2 for my ears, I was constantly craving more upper mids and I'm thinking that may of been what fatigued my ears and caused my ears to strain to listen to it's upper mids oddly enough. I ended up feeling it's upper mids was lacking even coming from the T1.3 which is well know for it's subdued upper mids, but somehow it just works for me on the T1.3 whereas I think it killed the S2 for me. Which is sad because I liked everything else about the S2's sound and it was one of the more musical headphones I've heard recently.
After recently spending time with the HD 660S, HD 660S2, HD 650, and the HD 600. I'm beginning to favor the HD 660S overall of the HD 6xx line. The HD 660S2 and HD 650 initially drew attention away, now I'm back to favoring the HD 660S. But honestly I think I like the HD 650 the second most of the bunch as it has different strengths.