Variable Voltage Regulation (VVR) is a way of making the B+ on your amp variable like using an external variac only unlike the variac it doesn't affect your heater voltage. As you turn down the voltage on your tubes you drop the power but keep a lot of the characteristic distortion and sustain only at a lower volume. It works far better that an attenuator and is a very simple circuit with just a handful of components that are fairly easy to install if you have the space.

There are two types of VVR - a) whole amp and b) power tubes only. Whole amp VVR reduces both the output of the preamp and the power tubes and because the preamp tubes are also operating at reduced voltage, they are actually easier to saturate. If you regulate the whole amp the preamp's voltage is also reduced and the gain structure between the preamp and power amp is maintained. Power amp only VVR is when you regulate just the power section and keep the voltages normal on your preamp tubes. With power amp only VVR you will need a master volume to keep the preamp from overdriving your power tubes.

More detailed information can be found in the VVR forums here:

http://www.wattkins.com/vvr