You don't need any service manual and strictly, not even a schematic (although it helps) to change tone control and bright cap values.
You just read the current values and replace by higher/lower values and test.
That said, those are classic amps, tone was reached afcter endless experiment and was tweaked to death, at the factory itself, until present values were found.
Modding them will certainly change sound, one way or the other, but I guess sooner or later you'll return to stock.
Have witnessed that way many times to keep count of.