Interesting thoughts, i look at it another way. I see it that the recording captures the 'natural music transient event' so I want that to be reproduced as cleanly as possible. I can see the draw to tube amps though, it took me a long time to get used to the Hd800s extremely clean , clear sound. Back on topic though my hD600s are in transit and are due to arrive tomorrow. Looking forward to hearing this legendary headphone
Studio recordings generally have the mic much closer to an instrument and when played back through revealing headphones sounds like your ears are where the microphone was placed. Played through speakers and the sound will travel via 'cushioning' air before hitting your ears. So there is merit to naturalising transient events for headphone use.
With the HD650/600s having a less articulate driver than planars and top level dynamics...I believe the grain comes from top end energy as a result of the driver trying...but not entirely succeeding, to track the electronic signal in the top end. Using a tube amp can synergise better with these cans as I believe they produce a cushioned top end transient event that the drivers can articulate more accurately. The result to my ears is a vast reduction in grain and a more realistic sound treble e.g. cymbals sound more realistic via a tubed HD650/600 than a solid state driven one.
On another note different tube amps...different tubes resonate (or track recorded ambience?) more prominently in different frequencies. This can be used to tune the sound not unlike speakers resonating in a room. The whole idea that tube amps are always warm is a load of rubbish.