In a nutshell -- Stan reworks the circuitry and power supplies. He mainly does this to the Adcoms 5200, 5300 or 5400. These amps were originally designed by Nelson Pass but quality was compromised to meet retail price points. He corrects these things and truely does redesign them, while eliminating unnecessary components in the signal path.
He is hard to get a hold of and does not use e-mail. You need to call him and keep trying.
History -- one of the founders of PS Audio, then created Superphon and the first dual mono preamps. Sold Superphon and started SuperMods.
He also does great things to CD players and his daughter makes very good cables. Hope this helps.
I highly recommend his mods. Good luck.