I also have both of them and I think it’s a perfect duo: 660s have more intimacy, pushed mids and more bass, while 800s is on a higher level of clarity, separation and soundstage. They’re both great headphones, but my path of appreciation was different.

660s was my first love as an audiophile grade device and 800s - one of the last steps in this hobby. Both of them are extremely comfortable, but 660s wins by a small margin - because of the headband and more “pillow around your ears” feeling. I would never sell these combo of cans, they’re a perfect couple.

IEM: Andromeda 2020
HF: HD660s, HD800s