Further adventures in 90s Casio MIDI gear:

CTK-601 keyboard demos (without DSP):
https://revenant1.net/casio/ctk601_demo0.ogg
https://revenant1.net/casio/ctk601_demo1.ogg

GZ-70SP demo:
https://revenant1.net/casio/gz70sp_demo.ogg

I got the latter working thanks to Casio's old SW-10 softsynth for Windows - the sound bank it uses is apparently just the entire ROM from the GT913-based MIDI modules (GZ-30M and GZ-70SP) and the WG-130 daughterboard. I've also been able to use the softsynth as a reference for improved mixing and envelope behavior.