I just bought a pair of these, so I can answer my own question. I thought they would be more detailed but on my Yamaha 'Natural Sound' receiver with 220ohm jack they are actually smoother than the 600's. My Hifiman HE-500 had this same sonic character (smooth), but I've had plenty of headphones that are very detailed on my Yamaha like Sony's and AKG's. Actually, if my 600's had any more detail it would be too much.
Anyway, the 660's sound like a cross between the 600's and 650's. They have more bass than the 600's but not as much as the 650's. I do prefer the bass control at -1 and on darker recordings, -2. I listen to mostly vinyl rips and vinyl does have more bass...depending on the system of course. With CD's the tone controls can stay flat, with the occasional -1 bass setting. (Tone controls definitely come in handy!!). Also, the soundstage is noticeably wider, which is nice.
I don't think these cans would sound good on bass heavy receivers like the early Sansui's. In fact, I don't like the sound of my 600's on my Sansui 800.