Actually the power scaling circuitry isn't at all complicated. It uses a simple high voltage MOSFET regulator for the B+ with an optional tracking bias supply for fixed bias amps so that the bias goes down as you reduce the B+ so that the amp maintains correct operation. The methods for doing this are fairly obvious and circuits are given in TUT2 and TUT5 (as part fo a project). TUT4 has a rather long and self serving chapter on power scaling which really doesn't give much more usefull info than that in TUT2 and TUT5.
The one problem with power scaling is that it uses solid state components that are more suitable for mounting ona PCB than hand wiring on tag strip etc, however these aren't insermountable problems.
Some manufactures have incorporated a variac into their design, however this only works if the filament supply is run off a separate transformer (and indeed any other supplies for channel switching etc). As the Variac + additional tranformer/s are quite bulky this method would be more difficult to retro fit to an exisiting amp.
Power Scaling is a trademark of London Power so you can't build and market an amp as Power Scaled without licience from London Power. However, my guess is that any patent of this technology would not stand up in court, so I suspect it won't be long before this type of control starts to appear on mainstream production amps.
