I bought this pair of headphones as a step up from a lower price bracket. I also own modded Fostex T50RP and Hifiman HE400i 2020. They were bought for home music enjoyment and I have been listening with them on an Ifi Zen Can and Schiit Magnius amplifiers. On both I find the sound extremely immersive - feeling like it is all around me, with enough separation to give interesting and absorbing detail but not a great deal of width. I am finding the most fun listening to rock and metal on the warmer Ifi which to my ear these headphones work particularly well with. Classical music can feel a bit heavy and claustrophobic on these for me but is still very enjoyable for the right composers!
Comfort is excellent even though the clamp is a little strong. I am not sure I'd want to wear them for too many hours. Even though they are open back they feel very enclosing.
They feel extremely well built, not heavy, and with a sleek, spare design. The pads are soft, and pleasantly textured. The whole headphone feels good to the touch and the materials refined and reasonably sturdy . The biggest drawback for me was the included cables being so long. If you listen at a desk be prepared to need to buy an alternative set.
I like them, a lot. They do demand that they be listened to so maybe less suitable for chilling out but for active listening they are wonderful. Very happy with my purchase and I would recommend the HD 660s to anyone looking for a (slightly) warmer, detailed and exciting headphone.