Shuttle 9.0 would work well, I'm sure. Shuttle 6.0 would probably do well also. When I used to use a Peavey Combo 300 on stage, occasionally I would have pretty severe "headroom" issues when slapping both upright and regular electric. I partially solved this by just going direct to the board and using the better quality stuff we already carry around in the PA racks, and getting rid of the old Peavey amps. I also lowered the action on the upright, and started trying to play with a lighter touch, so the transient signal when you slap isn't quite so mondo loud. That helped, also. It's not a volume issue, as much as a distortion issue.
I find the small size of the shuttle pretty darn attractive...as well as the fact that our resident amp expert PBM has said that they are well built...
I notice he has been silent regarding my other choices, so maybe that should tell me something.
I'm 100% sure a shuttle 3.0 would also work well, if it were not for the rockabilly slap stuff, and the R & B slap stuff we do. On stage we're just not that loud.