I don’t think it would add anything to your setup except, well, more FM. It’s a good little box, bit if you haven’t experienced the Digi line yet… It’s a bit of a let down in some ways compared to even comparative antiques like Monomachine. I don’t know if this is due to product immaturity, if they’re just deliberately treating the Digi line more like ‘groove boxes’ and leaving some of the more advanced configurations to the big boxes.
Monomachine: I kindof prefer Monomachine’s FM, but it’s easier to push Monomachine’s FM into weird percussive territory that still sounds really neat, and I like that. But I always have to have the manual close by to remember the controls and differences between the Monomachine’s FM machines. Digitone has a much more informative and dynamic display.
Volca FM: The Volca FM has those special “carrier attack/decay” and “mod attack/decay” controls on its surface. Those kindof remind me of the Digitone’s simplified pair of FM envelopes for its A and B-pair operators. It’s easier than going into the nitty-gritty of six different multi-stage envelopes, but the Volca FM does at least give you the deep editing of those envelopes (if you want it); but Volca FM also gives you relatively little in the way of visuals.
Reface DX: I’ve only used a friend’s Reface DX briefly, but I really liked my experience with it. Digitone and Reface share the simplified 4-operator engine (which I don’t mind - I’m much more fond of weirder FM like on Monomachine than having every FM synth be a DX7 clone) and have good on-board effects. And Digitone and Reface DX have pretty good graphical representations of what’s going on. I was really wanting a Reface DX, but it didn’t quite offer enough over the Volca FM for me to add it to my collection. But I bought Digitone immediately because of the Elektron sequencer and not needing to configure Monomachine or Octatrack.
So no… I don’t think it would add too much to your setup. Especially if you have Analog 4 too. I’d rather have Digitone instead of Reface and Volca FM (although I’m keeping my Volca), but if I had a Reface I’m not sure I’d bother with trying to sell it to get a Digitone. And if I had the Reface, I’m also not sure I would have bought the Digitone as swiftly as I did.