To me there is no comparison ... HD 650 is better on everything smoother , more detail , more polished , way better timbre and natural sound.
58X vs HD 660s Its a night and day difference , the only common is the sound signature ... Everything else is so much better on 660s , Imaging , separation , smoothness ( compared to 58x ) , tighter bass, faster way faster when a lot of different instruments are playing .
I have 2 years of side by side listening tried on 3 different amp dac compos from ifi , Fiio, Harman.
Some reviewers say that 58X are 95% of what HD 660s , i understand that because reviewers just listen some songs for some hours and that's it , but the truth is that if u have both of them and listen them for days , weeks , months .... then u gone understand why HD 660s is way better headphone .
KingKatoTo me 6XX and 650 sound the shame... both are amazing and smooth , way better in my opinion from HD58X.
About HD660s these have more clarity on everything in series , better separation , better imaging on all HD6 line .
I love HD 6 line i have all of them , So to me HD600 for neutrality , HD650 for smoothness , and HD660s for faster , clearer , better separation and imaging ... Are the HD660s better than HD 650 -HD6XX ? If u are looking for smoother Headphone NO HD 6XX -650 are the king on that... If u looking for other things yeah its Better.
HD58X remind me the resolution of my HD 5 series headphone but with 660s sound signature , but side by side is the weaker of them all for sure.