Depends on your definition of "backup." I've had various ones over the years. If you're not worried about budget and are thinking D800+, Puma, etc, then you're not necessarily looking at "backup" so much as another possible "first grab" amp with a different voice. I've owned/gigged a lot of the usual suspects (D800+, D800, Puma 500/900, GB Shuttle 9.0, Streamliner 900, GK MB Fusion, MB LMTube, TH500/350, GLock BlueRock, etc.). It really depends on if you're looking for relatively uncolored or something more voiced. From what I've heard the MG800 is in the ballpark of the Shuttles, which I found fairly uncolored. I think the TH500/350 are great amps but in my hands they were a one-trick pony - that more vintage "bit of hair" vibe. Other guys claim you can clean them up but when I did I found the magic went away. The Pumas tend to be reasonably neutral as well, and the taste knob is a nice voicing tool. GK has a baked in tone (again, in my hands) that is pretty edgy. I could eq it out, but then I didn't have a lot more room to play.
I've settled on the D800+/D800 as my main/backup. It is voiced a bit darker than the Puma, but I don't really dig a lot of top-end sheen. It has a great eq section (especially the 800+), and the DI is good. I keep the D800 in my car, the 800+ goes on most gigs. When I'm lazy I'll just use the backup. The advantage is that I know exactly what tone I'll be getting and how to push it around given the room I end up in. There is a good argument to have something different as a backup so you can change things up, but thats another reason to buy a 3rd amp