I like my R70x more than my HD 660S2, HD 660S, and HD 650. Yes the HD 660S2 has a fuller richer sound, but it doesn't sound as correct as the R70x and doesn't have more bass extension than the R70x (just more presence). The HD 660S2 like the HD 660S has a slight lack of effortlessness to their sound which can be a bit bothersome with longer sessions, this is the main drawback of the HD 660S/S2 for me. Honestly since spending time with the R70x on my Lake People G111 MKII (first SS amp I had that truly properly drove the R70x) the HD 6xx line has mostly been ruined for me.
The only headphone I like more than the R70x is my Beyerdynamic T1 Gen 3, which while I may get some flak for this opinion considering it's unique tuning, it's the most organic and natural sounding headphone I've ever owned to my ears, it just sounds right to me. I've owned the T1.3 for over 3 years now, the R70x was the only headphone that truly took a lot of time from the T1. With the HD 6xx line while I like them with the HD 660S2 being my favorite, they're lacking that little bit of something that prevents them from being truly great for me like the T1.3 and R70x are.