Oh and also some of the best Synths on iPad have touch screen interfaces that do many of these features as well for those who want to be a bit more mobile. Some standouts I might recommend are Propellerheads Thor, Animoog by moog music, and several of the Korg line - iM1 is quite powerful. There are a bunch of others too, but these might at least take you down the rabbit hole if you decide to go.