I first heard this mod in the early 1980s and didn't like the sound so I never installed it in my amp. It's based on the idea first used by the Mesa Boogie Mark I amp, and it's fairly simple to do on a Marshall 1959 Super Lead. Lots of variations have sprung up on the web to make it switchable and to make it sound better.
I did play around with the idea of cascading a tube stage when I built the Tube Preamp--that can be configured to put one tube stage in front of the amp's first preamp stage, and it doesn't require internal modification.
If you'd like to try it, find the point on the circuit board where the two 470K mix resistors meet, and isolate them. Disconnect the input to the grid of the Normal channel preamp tube, and run a wire from the Bright channel's 470K mix resistor and 500pf bypass cap to the tube's input.
The reason I don't like this mod is the sound is buzzy, which is what happens when you slam a tube with too large a signal. Instead of trying to get all the distortion with two tube stages, it seems to be a better sound to use several stages, and gently push each one. The Peavey 5150 amp uses up to 6 tube stages to build up its distortion, my homebrew uses 3. But back at the time this mod was popular, this was a new sound and was just being explored and tweaked.