i've always found that if you have a great rehearsal before a show, then the show will suck.
if you have a terrible rehearsal before a show then the show will rock...just works out that way...
i have no cosmic rhyme or reason why...just an observation!
I think there is a very natural reason for this: Nail rehearsal, you might get complacent and forget to focus at the gig. Struggle at rehearsal, and you'll do whatever you have to NOT play that poorly at the gig ("man am I glad that wasn't in front of people"). Pretty good incentive to be ready.