What does tube bias do for sound?

I have a VTL MB750 Signature amp with 12 6550C tubes per side. The recommended bias is 30ma, but I was wondering if ayone knew what would happen if I went higher or lower.
I think running the bias too high will shorten the life of the tubes for one thing, which might be a bit of a concern with 12 PER SIDE !! In my experience, I have found that a higher bias results in a crisper sound, while a lower bias results in a warmer sound. I'm sure this varies from amp to amp, but that's what I found on two tube amps I have owned. In general, I think it is best to set the bias at the recommended amount. You never know if you are straining the power supply by setting the bias too high.
In addition to what Audiolover posted, I can add that if there is a single bias adjustment for all the tubes on a side, then they might not be linnear if the bias is adjusted far from the reccomended setting.
Grid-bias voltage is almost always *negative* in audio amplifiers. Negative grid voltage inhibits current flow through the tube. Thus, turning the bias pot so as to make the bias voltage *less* negative will have the opposite effect and will *increase* the idle current. When idle current is increased, the amp moves towards class A operation. When decreased, it moves toward class B. In between it operates in class AB. In addition, when the bias voltage is less negative (say -40V instead of -60V), the driver stage does not need to swing so much signal voltage (to overcome the bias voltage). Thus, in class A operation, both the output stage AND the driver stage operate with less distortion. You just need to make sure you don't over-dissipate the output tubes by flowing too much idle current (plate dissipation = plate volts x idle current). If the tubes get too hot, all bets are off, and the sound gets glassy and hard.
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.