Crud is great sounding app for Dubstep/DnB bass lines.
It comes with nice looking interface and very simple controls: 6 knobs to control main parameters that depend on the preset. The app comes with nice 50 presets.
What is great it has Audiobus support. The developer claims MIDI support but it seems that first version of the app doesn’t work with Virtual MIDI.
Here’s the short video demo of the app:Free!