Roland Sound Canvas? Got to have at least an Air to get this to work from what I hear. I'm still sporting a plain old 2 so have not tried it yet.

ThumbJam is very expressive directly on the iPad interface, but has a bit of latency with MIDI for me. iM1 is wonderful but easily over-taxes the older iPad CPU.

One other that I like is Bismark bs-16i: an eight-part multi-timbral ROMpler with fairly decent sounds and rock solid stability at a reasonable price.

Gig: VR09; Numa Compact 2X; QSC K12; JBL PRX615M
Others: Vortex2; Deepmind12; ES100; MS2000R; Fantom X7,
JX10, S550, VK1000; EX5, MODX7, SY55 & 77.