I’ve tried a lot of stuff, and the HD660 is my current #1 rec for people looking for an overall good sounding, comfortable FPS headphone(that’s relatively affordable). I felt they were slightly less accurate than Sundara, but both more accurate than the old AD700X/900X. Now, I’m pretty much in your position with what headphones to try next, that are serious-level for FPS. Obviously HD800/S is #1 on my list, and Focal Clear also highly interests me. Other than those 2, I’d try some general end-game headphone for music, like SR-009, Utopia, AD5000X, LCD-4. If you want to test an IEM out, I have been exceptionally pleased with how good the Shuoer Tape Pro sound in-game(for only $129), but they may not be comfortable.
Going in the opposite direction, I felt the 300 R have very good accuracy, “good” overall sound for $199. I still found them to be slightly bright and very dry, like their DT siblings, but hopefully the brightness will reduce as the pads wear in. I just can’t stand that typical DT sound, or how they fit and feel, so if you are a Beyer person, they are a good choice in the lower budget range.