Surprisingly good for closed-back
I own a pair of Sennheiser HD 650s. These are open back cans that articulate as well as anything under $1000. I have owned the KRK headphones for even longer. When I first listened to the Sennheisers, they sounded awfully familiar... like my 8400s! I did a bunch of A/B tests on different genres of music, and I was forced to admit that the difference between the KNS 8400 and the HD 650 is pretty subtle - the Sennheisers sound slightly more smooth in the midrange frequencies, but I have to switch headphones very quickly to discern it. I use the amp in my id22 interface, which is no slouch. I'm beginning to wonder if there is any point to my keeping the Sennheiser headphones, other than they are more comfortable - the headband extends over my big head further. Practically, the KRK headphones have a single-sided detachable cable and an inline volume control going for them. Closed back is better for tracking and privacy. And they are less than 1/3 the price. The Sennheisers may do better with a high end amp and DAC, but for my humble little project studio, the KRKs are nearly identical in sound. You can't go wrong with the KNS 8400s.