If you like the sound and can find it for under $400 US, the Dave Smith Evolver is great. Deep MIDI support, lots of routing and modulation options. Including ability to easily route common CCs (breath control, etc) within the synth so it even works pretty well with the more limited CC sequencing on the Monomachine. And it has MIDI Thru - something all too rare on smaller synths (and even a lot of larger ones). Supports program change.
I've got the Korg Monologue on pre-order and hoping it shows up around the same time as my Octatrack. Korg hasn't published its MIDI details yet though, which is a bummer, so don't know if it will support program change but should hopefully have decent CC support. And at least it does have patch storage which the Volca Bass and Keys do not.