I use a powered USB mixer, powered hub, a USB/midi cable, and a couple USB controllers via the CCK. Works flawlessly, MidiBridge sorts out the routing. I’m doing this with the Monomachine, but that shouldn’t matter. MNM set to autochannel, so I can sequence (or play via the keyboard) internal and external synths simply by selecting the track.

I’ve got a Novation X-Station on the way, which should simplify this setup, but the main things to look for are class compliant devices, powered whenever possible.

The biggest issue is that I can’t power the iPad while using it, which limits how long I can really use it as a production tool (and is also another reason I’m buying the OT in a week or so, so I can sample from my apps). If you’re using a Lightning enabled iOS device, options are pretty limited right now, so you may just want to hold out until NAMM to see what comes out.