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.
