Don't underestimate the amplification. For example, this last week I've been listening exclusively to the HD650 being driven by the TEAC HA-501. The sound was very crisp, precise, and transparent. My ear burned in nicely.
Then, last night to change things up, I switched to my DT990 600 ohm being driven by my Woo WA3. The DT990s traditionally have deep, tight bass and sizzling highs. Well, compared to the TEAC HD650 combo, the DT990 and WA3 was extremely warm, laid back, flabby, and wooly.
Amplification makes a big difference. In last night's example, the DT990 was more laid back, mellow, and euphonic.
Yeah, amplification means plenty.
I have had tubes that needed longer to burn in than some headphones....I think many people forget that about tubes.....give them time before you make any harsh decisions.