Okay, so I'm obsessed with Billy Gibbons, and I've done hours of research on this very subject. The following will sound like a joke, but it is 100% the truth I swear.
I actually found a phone number for an amp tech on the Texas / Mexico border. The guy I talked to was shocked that I got his number, apparently he's been retired for a while. But anyway, he was the guy that did the mods for the "Rio Grande" amps. He explained to me that they were nothing more than 1969 100 watt Marshall's redone with new tolex, logos, etc. No internal mods were done. He also told me that Billy used to collect Fender "plow front" Dual Professionals, and had like 18 of them, and frequently used them for recording. He also used some Alamo amps, (the little green 1x12 model), and sometimes a Tweed Deluxe, and a 100 watt Marshall with a 4x12 cab. I thanked the guy for his time and hung up.
Then a few weeks later, I tried to call back and the number was disconnected. It really sucks, because this was the best source of Gibbons info I've ever had, and teh guy sounded very legit.
But as in all things concerning Billy Gibbons, take it for what its worth.