A backup is just to get you through a gig, you don't need another big valve amp for that.
I do happen to use a valve amp as a backup, but its only 180mm x 110mm, weighs around 3kg (if that) and fits in my gig bag.
If I wasn't building valve amps myself, I'd probably get a POD or something as a backup to DI straight into FOH. If you have no monitors, even something like an old Marshall Valvestate 8100 head will serve as a backup - Chuck Schulder of Death used these as his main amps fwiw.
