What does tube bias do for sound?

You have one of the great tube amps but the VTL circuit design is specific for a slightly lower than normal 6550 bias in class AB. Your amp should also allow for true class A operation at half power. I would strongly recommend that you stick to the VTL specs and pick AB or A operation based on your listening.

Thanks for the advice. Looks like the bias "effect" is a function of the amp design. I may keep it the same and look for other tweaks to work on.

Yes, higher bias will increase the warmth, and decrease the tube life....sometimes drastically. Lower bias will make the sound less sweet, but will tighten it up and make it more dynamic. This is my experience with my old Dyancos.