IMO, the answer is, "It depends..."
Lots of good suggestions here. If you have to stay digital and you're on a budget, I would look at a used Atomic product as backup.
Certainly, the answer depends on how you use your AX8. If your gig requires a different preset with multiple scenes and an array of cosmic effects sent to FOH, some form of digital is probably the direction you'd need to go.
I look at backup as something that will get me by for now, even if it isn't perfect.
With my setup: AX8 split to power amp/cab on stage with a line to FOH, I could use my small, 20 watt Jet City head as backup and survive just fine. I would lose some versatility, obviously. But just set the head on my existing cab (mic'd). Maybe have a designated backup board with a delay, chorus, and drive pedal. And that's it. (I didn't sell all of my old gear once I got my AX8 so I still own all of this stuff.)
Even if you had to buy these things used, it wouldn't be much $$ and it's just there for you, easy to transport. I could do this and with what I play, the audience really wouldn't know the difference.
If you can't survive without having the same results as the AX8, I would knuckle up and just buy another AX8 and be done with it.
Good luck!