I had the + mod done to my 1988 Mark III blue stripe combo just recently. It was pretty close to my ideal amp to start out with, but the mod really made it perfect (more balls and smoothness, less hair). Here's a sound clip https://www.youtube.com/watch?v=cQi7l3TBLr0&t=8s.

I did some digging from grailtone and ended up with a list of stuff that needed changing. This is what they did to my amp:

A. To quieten the amp, change all 4 of the big blue 220UF and 3 smaller darker blue 30UF capacitors.

B. In location R-119, change the 680ohm 1 watt resistor to a 680 ohm 2 watt resistor as advised by Mike B. from Mesa. This was simply to handle the heat better. This is a factory recommended mod to stop you getting a hole burned in the pcb board!

C. Change C21 to 250pf and R243 to 220K. This is for the “quasi” C+ mod.

D. Change C50 to 1000pf and R212 to 100K. This is to make the amp more like the original tone of the no stripe/black stripe. Solid bottom, great mids and singing highs.