I could never really warm up to the HD650 on an OTL tube amp either; it sounded very boomy, and had no idea why people like these things. On the solid state Matrix M-Stage with the Tam Audio class A OPA627 mod they're a completely different headphone. They're so tight it's like getting hit in the head with a foam bat. In fact, they're almost too far the other direction and it's difficult to read, work, or do anything else while listening. Undoubtedly this has to do with the impedance hump in the low mids, which is likely inductive, and requires dampending. I get the impression OTL tube amps aren't very good at that, despite output impedance and power ratings.