I also don't quite agree with this. I have noticed remarkable improvements with my LG (RCA Coin Base 6SN7s, Thanks Morbid!) paired with the HD800, but the brightness is still there on some tracks. But IMO, how bright a headphone is completely subjective to the listener, ergo a change in amplification may change that, but more than likely not.
Disclaimer: I'm quite sensitive to high amounts of treble, so something that sounds bright to me, may not be exhibited in measurements or for others.