I still have my 580’s, 600’s and 650’s, and they are all in regular rotation. I liked the 660S, some of the time, but not as consistently as the 300 ohm siblings. I really think it’s a matter of amp matching. I know, I’m a broken record with that, but it’s true. This line of Sennheisers is very picky about amps. I guess the 660S may be less so, but I couldn’t find the exact right combo to make them clearly better than any of the other three, so I let them go. Right now my favorite of the bunch is the CC modified marble 600 with Dekoni hybrid pads and a custom made mogami wire with furutech connectors all around. Hard to beat at any price, even though tomorrow I’ll say I like the 650 best other days I wonder why I ever tried to replace my now 25+ year old 580’s. That’s why, as advised, once you’ve got ‘em, you gotta keep ‘em. Otherwise you’ll just go out and buy them again anyway. My perennial favorites, even over dozens of other fancier contenders with big reputations. The Senns have a way of subjugating themselves and allowing you to sit back and enjoy music, as differentiated from listening to headphones.
YMMV, IMHO, just my $.02.
