It's my understanding, but I stand ever willing to be corrected, that when you change the Soloist's gain setting from Low to Medium (or from Medium to High), then adjust the attenuator to restore the original volume at the headphones, you are not changing the sound signature at all.
The Soloist's gain setting is a multiplier of power. The attenuator is a divider of power.
So, increasing the gain then rotating the attenuator counterclockwise (for more attentuation), to restore the original volume level, accomplishes nothing other than shifting the range of volumes covered by the attenuator.
It's best to leave the gain set on Low, increasing it to Medium (or to High) only if you are unable to achieve your desired volume with a given headphone, when rotating the attenuator fully clockwise. The lower the gain setting, the more gradual will be the 24 steps of the attenuator - allowing you to more finely tune the volume level.
All that said, if you (or anyone else) is hearing a difference in sound signature, such that the Soloist sounds "better" when operating at high gain, I have no explanation - but I believe you.
For what it's worth, that's how I've always understood it to work too. You should always use the lowest gain setting that will allow you to achieve your desired volume levels. If people are hearing a sonic difference it may just be that the Amp is reaching it's distortion point faster on the dial.
Edited by bareyb - 3/17/13 at 7:22pm