Audio Research VM220 tube monoblock amplifiers

I paid 4k for a pair with only 150 hrs on them. I am very happy with the sound. Does anyone know how high you can tweak the bias ma on the stock 6550 tubes? Manual says 65 ma. What is a safe setting that gets you a significant change in sound but still gives you reasonable tube life. I set my bias at 68 ma. Should I hear a difference or do I have to go higher?

Set the bias for 65mV that's it. Going higher buys you nothing except decreased tube life. I tend to set em a bit lower actually like 63mV.

Do you get a change in the sound at 63, or are you setting things low to get more tube life?

Does not change the sound. I normally set bias at 120Vac line voltage