In terms of MIDI grooveboxes, I see Polyend Tracker and Square Hapax are interesting, but they are not cheap either.
As a result I've cheaped out and when not using my beatstep to control the stuff via my DAW I have a Korg SQ64 but its a bit unintuitive flow-wise in terms of live jamming. I end up just using my TR-8S with the 2600 or 303 side-chained through it.
Analog 4, Analog Heat, Analog Rytm...