I have an SD4 and had an SD2 for a long time. Use with midi accordion..tried with my Tyros but they didn't play nice together..
having trouble saving midi transmit channel info in tyros to match Ketron. Generates default sounds. No option to save tx setup..re-enter..re-enter...Conversely, receive channels save between power on/off..something quirky in keyboard with age. Preferred Ketron sounds..and modules fit in rack. Made a slide out partial shelf for sd4. I think Korg is more compatible w Ketron modules?? Anybody confirm?

I loved my SD3 for midi accordion independent of keyboard (w rhythms/drum), but separate stand and setup wiring and time gave way to convenience. Had Solton MS40, Ketron X4 before that. I often think about Varranger..that requires adding my tablet and interface to mix..

I think SD2/1000 are such a good option for such a small footprint in rack or velcro-ed to keyboard top. If support was more widely available, I would just get a Ketron keyboard..no need to re-visit that..