I've never heard the HD660S, but bought a used HD700 with the intention of running them with Oratory1990's EQ setting, because of:

• low unit-to-unit variation
• low measurement variation depending on the placement on the head
• sound doesn't change that much with pad wear (reportedly HD6XX series pads change the sound after just a year).

That did not work with my K702, my unit had a different placement of peaks and dips in the FR. I settled on a bass shelf with them and was happy, though they do get fatiguing after a while.

With HD700 it worked well and the sound is very good. Has the non-fatiguing tonal balance of K371 but with more sparkle and soundstage.