A couple of notes about Plexis, powerscaling, and such.
1) If it's a Plexi, you might want to try a post-phase inverter master volume. No, it's not powerscaling, but the tone is 50% there. Basically, it allows you to overdrive the preamp and phase inverter, but not the poweramp. The key with a Plexi is that comapred to other amps, phase inverter distortion makes up a larger part of the signature sound.
2) As for power scaling: London Power sold power scaling, sells power scaling, and then there is power scaling. The gist of the method is the same, but their is different implementations. What powerscaling does is to scale the voltage going to the poweramp tubes. For fixed biased amps (as opposed to cathode biased), it also needs to scale the bias.
London Power has sold kits (an external box that wires into your amps) and schematics. Even then, they've changed the way they scale (AC vs. DC?). I installed an external scaling module into my Metropoulos Plexi. It's not the neatest install, as it is an external module tucked away inside the cab, and I need to reach around back and inside to twiddle the voltage knob. I installed a PPMIV with the knob coming out of the second speaker jack. In this installation, it acts like the drive compensation (as you turn down the voltage, you also need to turn down the signal sent to the poweramp). They've since change the kit and schematics (DC to AC?) which is much less likely to blow out mosfets, which was an issue for some folks.
Having said all that, I would have preferred to used the schematics (rather than the kit) and install it into the original build, so that I had an cleaner install (e.g controls in front). But, that also takes some planning and scheming on your own (or by your amp tech). It's not plug-and-play: you need to understnad the circuitry before you jump in.
For what it's worth, the 18watt VVR mod is functionally equivalent to powerscaling, except that it does not change bias (cathode-biased amps like 18watts automatically change their bias). The magic of powerscaling is that it also automagically adjust the bias -- which is NOT what a variac does (despite the Harmony Central claims).
