You have not said what the amplifiers are, only that they are monoblocs. It seems that they are Paramounts from context. If they are not, then please specify and I will clarify.
Paramounts have hum balance pots on the top of the chassis. These are there to minimize the hum from the filaments of the output tube. For best results, let the amplifier warm up for a half hour or so before adjusting. Ideally, the input should be shorted for this adjustment. You can adjust by ear, but for best results, put a volt meter set to read AC mV on the output. The center of the pot will be where it works best, work it one way until you hear/see hum, then back the other way until you hit hum, and so forth until the hum is minimized. You may need to touch up this adjustment as your tubes age, and definitely if you change tubes.
Paramounts also have a potentiometer on the driver board, which sets the plate voltage of the driver tube. I do not believe this is what you are referring to. This would need to be adjusted if you change driver tubes. Consult the manual for more information.
Is your isolation transformer a variac...? What is your incoming AC that requires reducing voltage? The power transformers are designed for 120V and the circuits are tolerant of some variation.