The best Bootlegs are always done when the "Sound Engineer" splices a direct feed from his board to a recorder, usually Reel to Reel, although nowadays a laptop might be the best choice. You get almost studio quality from this kind of bootleg, unlike some which are just done by someone holding a microphone and recording the concert.
An old friend of mine collected 100's of bootlegs, one in particular was terrible, a Tom Petty concert, and the sound was the worst I have ever listened to. But some of his others were very good, of course those were all vinyl albums, perhaps things have changed for the better now as far as quality goes.
I never collected bootlegs as a rule, I always bought the studio release back in the day.