When I plug the HD800 to the Asus Essence STX, it has too much treble and clinical. When I plug them into the V-DACII paired with a Lake People G100 amp, it sounds very neutral with almost no treble boost. When I plug it into my Onkyo stereoamp, it's somewhere in between but still sounds good. Only the Essence STX makes it sound too piercing (and missing texture too)
HD800 changes surprisingly much.
+1 on the initial part with the observations, just the conclusion has it backwards ;-)).
The HD800 doesn't change but it reveals the sound of each amp.