I used to use a Mesa Roadster combo for shows, and would take a Stiletto head with me as well, just in case. OK, it's not the same sound, but in an emergency I could just plug it into the combo's speakers and muddle through...
well, I went out with just a Kemper, no backup amp. My car was loaded with 3 guitars, pedalboard, wire bag, Kemper, guitar stand, 4x12. I knew my other guy was bringing 2 stacks so I was good for an emergency.
Next gig I may go with a 2x12, the Kemper doesn't need much with the di going to the PA. It was impressive!
I should bring my other head just in case but I usually just bring my head and cab and a guitar for each tuning... Hopefully I'll start having backups just in case but as of right now I haven't had a problem
>.> <.< >.> knock on wood...