Hi,
The reverb mod (a single resistor solder from memory) was great on my red stripe. Made the reverb sound better and removed a background buzz. Sorry I don't have a diagram- it died with my hard drive a few weeks ago. It should be easy to find.
Another mod done to mine, a 60 watt, was done to enable El34s to be used, a la a simulclass. I think it may be a cathode bias switch (not quite sure, but the tech labelled it 'Class A). I use it switched on with 6L6s and it makes the amp a bit more dynamic. I don't think this is relevant if your head is 100w.
I can also remember trying a very simple mod that involved wiring a resistor within a patch lead and running it through the effects loop to tame some high end shrillness. As I got better at tweaking the amp I found I didn't need it (In any case, running a pedal or two in the effects loop probably has a similar effect.
The best thing I ever did with my Mark III, however, was to use an attenuator and keep the master volume on about 5 (as I've mentioned elsewhere). I know it's not really a mod, but it had the single biggest impact on my tone.
Good luck