Hi Michael; congratulations on geting your amp back in good condition sounding good. Yup, the GII was originally fitted with a pair of diodes; nice uptick on the 5U4; that one can handle the voltage characteristics of the 7591s and give you some nice saggy shaggy ooowaaaooowaaa. Easy for me to like it; I don't have to play Shea Stadium.
At the end of the day, I think it's about volume; the 6V6s / 7591s just can't stay with a bass amp and drums at higher volumes. Not the easiest way to get it done but I'm not sure there's a comparable, medium-power, as sweet-sounding a 15" combo tube amp that can be had for much less than what you have in yours not to mention that yours has a renewed, 20-year lease on life. I guess you coulda bought a Clark or Victoria but they would have been 2 - 3 times the cost used and, if bought on eBay, no guarantee that they wouldn't have needed a trip to the vet for shots either.
A final point, your amp uses a 7199 as the output driver. The 7199 is sort of on the expensive side; most of $50 for a JAN Phillips, less for a Sovtek. If your tech didn't do it and you get interested, it'd be worth the trouble to pull the 7199, and the others too, get some contact cleaner and a soft brass bristle brush, and clean the light coat of corrosion off that usually forms on the pins. If you don't have another 7199, begin to suspect yours, or want one for a backup, I've got a Sovtek 7199 here doing nothing; let me know if you're interested. John