Well, pretty much, if you drive the gain real hot, you're gonna have super hot output coming out of the pre. In a real Twin amp, you can do that because you'll begin to drive the tube power section, and it's likely to begin to break up maybe before any preamp clipping. But if you try that with an external solid state power amp, you're probably going to clip the input to that amp before you get the preamp to overdrive itself.
I do have ways in the preamp to drop in some resistors to tame the output level, and this would allow for you to really drive the gain and mids into a zone where you may begin to get some mush and maybe some audible clipping, but it wouldn't be much, and you'd need a pretty hot input signal. But really, this thing is meant to be run as a clean machine like the way Jerry ran his Twin preamp.
The topology of this or a Twin pre goes like this:
Input to tube stage 1 -> passive tone section with lots of insertion loss -> gain pot -> tube stage 2 to make up the lost gain from the tone section and then add a bit more gain on top of that -> big pad to drop the signal quite a bit so that the reverb can be mixed in - sparkle is added at this stage -> tube stage 3 to make up the gain lost from the pad and drive the reverb return - this 3rd stage is the output driver -> output jack
So I'd honestly have to check again. It's been a while, but I belive that if you're gonna generate any dirt in this circuit, it's likely going to be from hitting tube stage 2. If you crank the mids and the gain, you can probably make something happen there. But again, the resulting output signal will be extremely hot, so something would need to pad the resulting signal to make it usable.
I've also included two trimpots to tweak the reverb send and return levels so it can run clean and be gain optimized for how a player intends to use the pre.
So short answer, in general, changing the gain on this preamp doesn't change the tone much. It's pretty linear.
... and it's just like any other day that's ever been...