To be honest, there have been two fails of amps for me during the last 36 years. First one was a GallienKrueger 400 RB that went up on fire during the gig after 4 years of heavy use, second one was my beloved SWR Redhead, that made some "magic smoke" on stage, coming from the poweramp. The Preamp was still working, so I could end up the gig - without a sound out of my cabs. I did lots of gigs with this amp during the more than 16 years I played it.
As I purchased a used Markbass LM II three years ago and not using it often, I take this one with me for backup when using my fullstacks with GenzBenz, Ampeg or even old Peavey - Amps on top. You can get a used 500-Watt-Amp for cheap, and the Markbass amps are reliable, good sounding amps.
I believe, that just because of having a spare amp in my cars trunk, none of my old irons will leave me behind. I play in three bands with three different fullstacks, in my main band I have a GBE 1200, the other gets the sound from a SVP Pro + Poweramp, and the third one from an old TMax with two TVX-cabs, one 410'', one 210''.
But I did make bad experiences two times, and because it's possible to carry a small, lightweight spare amp nowadays, I do this. As the boyscouts use to say: Be prepared!
I purchased the LM II for bar gigs, wanted to use the LM II together with a SWR Workingman 212'', upgradet with Beyma - Woofers to 700 Watt @ 4 Ohm. But for the last two years my Promethean 5110 gets all the love for small gigs.
If there are more bands in the evenings billing, it would be possible to ask a colleague to give you his bass amp in worst case. But I noticed during the years that its me who gives his amp to fellow bassmen. And cables, batteries, strings, you name it.
Ciao
Uwe