actually a great thing to do is run the Marshall via a dummy load into one of them! If you are looking for a higher gain amplifier check out my review of the guitar amplifier.
I think the right overdrive pedal out front and the right speakers will do you far more good than most of the mods you could get done to the amp, at no small cost, and you will not have hacked the amp up.
It takes a lot of work and dedication for a brand to reach that level, but a few of them did.
Because of this design the Bright or "Lead" Channel responds extremely well to boost and drive pedals.
The guitar signal enters at far left at the High "Lead" Channel input jack.