I continue to be amazed how much better my HD-6xx sound out of this amp (running Siemens NOS E88CC, balanced output) than from the other amps I've tried with them (all SS: Bryston BHA-1, Violectric V280, Massdrop LC-X, Soundaware P1). I bought them mostly to have as a reference, since so many folks use them (or 650) as one of their 'comparison' sets in various reviews, but they never seemed that great. I got the whole '...they don't do many things very wrong', but the veil and the lack of bass impact made them seem ... lesser.
But out of my LP they are a revelation. Much better at both ends, without damage to the mids, which were always their highlight for me.
My ears, my chain (of course).