It may be because you are using a Schiit amp. (sorry old joke) Seriously though, I find the 650's are more picky with amps then most people on here suggest. Yes they sound good out of most dedicated amps but only a few really bring the best out of them. Tubes give them a massive soundstage but also can make the bass muddy etc (I'm talking about sub £500). For me Hybrid amps are the way forward with the 650's. Tube soundstage with S/S timing and speed keeps them in check.
^ I agree on all accounts.
This one I don't agree with . It's possible to make the HD650s sound fast in the low-end. Just need the right source/amp. A recable helps too (assuming you have sufficiently good gear to begin with).