Expanding on Jim's post above, it's crazy how ear fatigue can sort of make most modern high gain amps sound similar after awhile when they're cranked. I mean there are obvious differences with some but after about 5 hours of ear shredding madness, Diezels, Fryette, Peters, all started to sound somewhat similar while others had a very unique sound no matter what like Mark series for instance. I'm not stating that the other amps aren't uniquely voiced because they are but that insane volume can play tricks on the ears after awhile.
The other big takeaway from these fests for me at least, and it's a BIG one: speakers make more difference than any other factor, period. I play 30+ amps at these with 14 different 412 cabs. Take almost any 5 amps and play them into a Cameron 412 with H30s and for the most part they all sound somewhat similar with just minor differences but plug the same head into a Splawn with small blocks or a Mesa with V30s or whatever and it drastically changes the sound and feel.