The developers of Molten Drum Machine, give us their take on a subtractive synthesizer app. They brought some great innovation to it in the form of a step-sequencer for automation that is controlled by key presses for rhythmic playing. Impressively, this app came out of the gate with Virtual MIDI and a host of MIDI features; like "Learn". There is a lot to like in this app, with a wide variety of sounds from cool to gritty. The effects section is also well thought out and feature rich. See a full review here.
Disclosure: I received my copy for review from One Red Dog.