Great piece of kit
Classic sound, I'm currently having fun using this to push the preamp in my all valve Marshall TSL 602.
The first thing I've noticed, moving away from my Spider IV modelling amp to valves and using this to get my tone, rather than modelling circuit is just how much sustain I'm getting off my Les Paul Standard. I've never heard it sound like that.
I've gone back to the Spider and compared - although it's way more convenient to push a button, you can just hear the sustained notes drop off way before they should.
So overall, using this pedal to boost signal into a valve amp preserves the actual instrument signal, its sustain and clarity, but be prepared to be fiddling with dials for a while until you're happy.
Good overall build quality on the pedal, LED is notoriously difficult to see when lit and the battery connector is a bit cheap, but those are not big problems. Recommended piece of kit.