We can only speculate, but I'd like to know what would have been the deciding factor, like "do we ship one of our Twins to New York, or would it be cheaper to rent one?" C'mon man, we're talking about playing a grand total of 15 minutes, does tone dictate that we must spend every available resource? Obviously the entire rig was too big and loud for a soundstage, it was designed to be used onstage with the Grateful Dead. It stands to reason that if they shipped one of their own, someone must have been along to schlep it, right? For a spot on Letterman? Or would Jer have schlepped his own amp? I mean the guy was in his mid forties and at that point very successful, and a twin loaded with JBLs is VERY HEAVY, as some of you can probably attest. The risk of injuring himself and jeopardizing future commitments would have been too great for that, I think.
Something I noticed about the Golden Gate Park video was the first few seconds show Jerry tweaking some amp knobs. Wouldn't a Dead-owned Twin already be set for Jerry? Seems like that close to home he would have Dead-owned equipment on hand, but wouldn't it just be easier to play thru a Twin that was already on the stage? Backline companies always have a spare Twin on hand for guests or backup. It is THE industry standard working amplifier, after all.