HD6XX Update:
After several days of listening, I think HD6XX has had some burn in. It doesn't seem to "struggle" as it did before, if that makes sense.
Again, I dinked around with EQ. Part of me says to accept the sound signature as is, part of me wants better sound through software. Ambivalence. I tried the eq in Foobar Just to make slight adjustments. I looked at the frequency response curve on RTINGS and noticed it was depressed at 14khz compared to the Harman Target. I boosted 14hz and a few others which I thought brought the vocals out more. But then I asked myself, is this right? For me it felt like it was going against the intention of the recording. I'm not looking for super accuracy but I want to stay in the ballpark. I loaded up Peace and AutoEQ - after the burn in it was noticeably nicer with autoeq, without the perceived distortion. I'm learning to not heed the siren song of the V shape curve.
I know some argue that the Harman Target curve is v-shaped. Does anyone else use the Harman Target to tune their equipment or something else?