This unit is the opposite of what it says. It is not a power booster at all. What is it ?
It's nothing more than an attenuator. There are two guys in fierce competition selling the original device on ebay which is marketed primarily for the Fender amps. The idea is simple. Valve amps give louder volume than trannie amps, and due to poor preamp designs they are very poor at low volumes eg Blues Jnr, Hot Rod, Deville etc to use at home you need low setting which on the downside doesn't cook the valves sufficiently to give them the mild distortion, mild compression and harmonics that brings a dull solid guitar to life. Slightest touch of the volume control and you are suddenly at " 747 in a hanger" type sound LOL. So these guys invented these attenuators for amps that merely crank up the power at same time cranking down the output volume. This marshall type one is targetting Marshall or UK users and I think works in a different way to the others. Am not convinced the effect loop way is going to produce the right end result. If you have a VOX AC3o eg or a Fender Twin Reverb, those amps don't suffer from the Blues Jnr and Deville type preamp design flaws, as you can still play these amps at low volume with no volume control level issues. But driving your valves harder would benefit by an attenuator and your neighbor would benefit too. So why not get your neighbor to go 50/50 on the cost ?

Most AC30 new designs including Valvetronix 120 have this type of product built in, so don't waste your money if you have one of these amps, plus it's not going to be any benefit on a trannie amp anyway as there are no valves to drive.