I have had a bass die on me, wireless go flat and a couple of leads die. I haven't had an amp fail. if the amp failed, I would go straight to the PA, so not worried about a spare amp, it wouldn't be ideal, but it would work. I always keep a spare jack lead beside me in case of wireless failure, although since the first time it never happened again and I have lent it to the guitarist who's leads have failed a few times, and he never has a spare of anything.
I normally have a spare bass in case, and I have enough spare leads of most types for most eventualities, and a 9v and AA batteries.