Paraphrased from the Owner's manual:
Replacing the stock 75V regulator with either a 90V or 150V increases Power Supply isolation making the psu more like a source of pure Direct Current.  This tube replacement also lowers the gain of the input tube, dropping the power from 20 watts to 6 or 8 watts. But within that zone, you may realize better sound.  

Steve, which voltage do you prefer?