Functionality and headroom are the two key ingredients for me. I tell opening acts they can use my rig, but that I wouldn't mind playing over theirs either, and if they prefer playing over their stuff I just ask "no drop-outs?" and "is it loud enough"? I then tend to have the MarkBass with me in case all fails, but if the other guy's rig is ok, the MarkBass stay comfortably in the car.
Really, all that "I need (i) this amp over (ii) those speakers with (iii) my effects board or it just ain't me" is for whining guitarists. Always embarrasses me if I witness that and if a guitarist wants to lose my respect really quick all he needs to do is say "I can't play over this thing". Pansies.
This may be all fine and good when you're playing shows with people of your social-economic demographic vs broke-ass student hipsters. The amps tend to really take a nose dive in both quality and level of maintenance. You see a lot of really bad cabs, speakers that needed a recone last year and amps that haven't seen a tech in a decade.
I do like playing other people's rigs (because I wouldn't ever get to experience some cool stuff otherwise) but sometimes it can be a real crapshoot. I was really keen to try this dude's Acoustic at my last out of town show but it turned out that it was distorted and farty at 2 on the gain knob. Dude was in a punk band and liked it that way, but it just wasn't something that was useful to me. I made the best of it without comment or complaint, as I did with the solid state SVTs in the rehearsal space we've been using lately, but it makes my day when we get the Markbass room (not that I am a MB fanboy, my top 5 choices would be different amps altogether). ... that may be due to being newer actually; you never know about the gear in sleazy rehearsal spaces.
The flip side of this coin is the otherwise destitute musician who has an immaculate vintage rig and won't let anyone else touch it because it's the only thing of value he owns. I understand that (I've had gear damaged when sharing; usually drums), but it's still funny/annoying; flip it for a brick shithouse Peavey and buy a new pair of shoes and some dinner, dude.
