I've been pretty interested in iOS amp sims over the last few years but a lack of support to actually control these apps in a live setting is a stumbling block. If you use the bottom port for an audio interface you don't have many options for controlling your patches aside from using your fingers and those are probably busy already
. When Mainstage 2 was released as a stand-alone app that it sounded like that was going to be a pretty killer set up for replacing an amp/pedal board for live use. Problem with that was there was virtually nothing in there for bass players. You needed to have something else for amp emulation if you wanted some options ( i.e. Amplitube3 and the Custom Shop ). Because of that, I never took the time to learn how to use MainStage.
MainStage 3 was just released and now the bass support is "almost" on par with the guitar/key support. There is a now a bass amp designer, many pre-mapped patches ready to play and some really useable plug-ins that will let you get some really awesome sounds without dumping a bunch of money. There doesn't seem to be any "bass" specific pedals but many of the pedals work. The other studio plug-ins really work great in shaping or effecting your tone. The amp sims look like they are based on the SVT, B15 Flip and Mesa 400+ amps, with some different controls added. They all sound top notch though.
If you checked it out in the past but weren't happy about the lack of bass support you might be really happy with the newest version!