Argon, little brother to Xenon, is on sale for a limited time. If you want a step-sequenced subtractive synth on your iPhone, this is a nice choice! 3 oscillators, and lots of modulation options, make this a surprisingly powerful little app.
It has all of the features we expect of iPad apps, including CoreMIDI and a looper that can be copied with General Pasteboard. There is also support for the SynthStation25.