Most of the guys on the board who've played a 20/20 would say that the deep mod is necessary for this amp, and I'm one of those guys. My concern with buying an amp with a mod already performed is how good of a job did the person do. Was it done by some sh*tkicker that thinks he's a tech? To me an unmodded 20/20 is worth more than a modded 20/20 unless you can show evidence that the mod was done by somebody who knows what he was doing.
Anyway I'm rambling...I think a fair price for a modded 20/20 is no more than $550 if the work was done by somebody who knew their stuff. You shouldn't pay more than $500 for an unmodded 20/20, and that's assuming in both scenarios that the power amp is in excellent to near mint condition minus the usual rack rash.
By the way, the JJ's EL84's from Eurotubes made my 20/20 sound really nice.