My V4 had a master volume knob, proved essential to slam the preamp precisely at different volumes. There's a point just before break-up where the harmonics really chime, it's just not the same amp at a different preamp level. I've since moved on to an SVP-CL/IPR combo and I don't miss the tube power amp, since it still gets that preamp chime.
You can make a master volume without modifying the amp by soldering a 500k audio volume pot to a cable with a 1/4" plug on the other end, plug it into a preamp output on the back. The knob works in reverse, when the knob is off the amp is wide open, when the knob is on all the way it shorts the preamp so no signal passes to the power amp. You probably don't want to run the amp with the preamp shorted for very long, but when used as an attenuator it's safe for the preamp.