H660S2 is very well built transducer, it's quite clampy out of the box. The clamps can be eased but stretch softly over the metal band on both side.
Sound: music is warm with mids forward. Spiciness in the treble is not noticed. Bass is also subdued.
Note: My previous headphone are 560s, Sundara, Eligia and Clears.

Comparison:

With 560s: the 560s is v shaped compared to 660s2. Mids/vocals sound so good on 660s2. But the sub bass, mid bass, highs sound bit better on 560s. 660s2 suits clasical music while 560s suits rock, electronic and pop music. 560s also tend to distort with volume and eq, 660s2 doesn't sweat with volume and eq.

With focal clear: the focal clear is best in all genre no competition but vocals are slightly prefered on 660s2

With Sundara: Sundara sound very thin when compared but excels in details and soundstage.

Overall 660S2 or any of the 6X0 series headset are must for audiophiles. This headset sings with right vocal based tracks.

Sounds very enjoyable and open and also has good imaging and instrument separation and definition to the various components of music is great. More clarity than HD 650 and also warmer but crispy sounding. I have the ifi zen stack but I like it more with just the zen dac v2 balanced output. Great headphone under 30k if you like warm sound with vocal intimacy and details of instruments.

You will enjoy the music as these sound good but little tight fit heavy when compared to competition like Sony MDR MV1 (which I owned already) . It's good looking when we compare it to Sony ones but only problem for me is it's tight fitting and I found little heavy for long listening, I can't bear these or even sony more then 40 mintues max in one go. Price could also be little better. But it's worthy one if you want to enjoy music at its best at stage live performance rather then in your head like closed back. One of the best ones in open back but again little tight fit and heavy for me.