Skip to 7:04 since it's just bullshit playing and what's important here is Soldano thoughts. He starts with a story about EVH.
It seems like this would be the perfect kind of design for vintage style amps (mainly Marshalls) that rely on power amp distortion. I wonder why more Marshall style builders haven't messed around with it. It seems like making a variac knob where one could adjust the plate voltage would be awesome. I know Fortin does something like that. That way you can put the voltage up if you need the headroom and then lower it for to push the power amp easier.
Wow that is some cool insight into how the 25 is not just a dialed down Hot Rod series.