So i'm still waiting for my replacement amp (due tomorrow) however I wanted to add a finding to this base of knowledge.
The seemingly widely held belief that the Luxury system doesn't vary preamp voltage but allows the amp to control volume internally is incorrect. It works exactly like the Premium amp, 5.1 amp, and pretty much every single other amp since the beginning of amps - by modulating preamp voltage.
While I was testing the preamps yesterday I noted that preamp voltage does indeed vary with volume change on the head unit. That means that when you turn up volume on the head unit, preamp voltage increases and as a result, amplified voltage does too - resulting in an increase in volume.
I was suspicious of this claim from the start since the only other way that the HU could prompt the amp to change volume was through the GMLAN, and theres only a low-speed (GMLS) link between the two devices. Trying to communicate volume changes over GMLS would be terribly laggy and buggy. Furthermore, iDataLink has only ONE firmware that covers all "external amplifier" systems on all Tahoes and Yukons, regardless of trim, from 2007-2010. That same firmware works on both Premium and Luxury systems. The only difference between the scenarios is the presence or absence of wires - premium has no GMLAN to the amp, so obviously it can't be used for volume control, but both systems have the same (front, at least) preamp links.
It does appear however that there is a significant difference in the preamp used in both amps. If the Luxury amp is indeed a major leap in quality then it's likely that it has a much more sensitive preamp. That would also explain why plugging a premium system HU into a lux amp could fry the amp.
