Ok, I figured it out. The system I was testing in had an 83hmz fsb, causing the ISA slots to be overclocked. This overclocked DSP/ESP to a speed it could not run at. Ran a test with a lower system bus and the card now works with the ESP module installed.

Wow, that's interesting! I know the architecture of the SoundScape boards quite well, and the ISA clock is nowhere near to the effects processor. Basically everything on that sound card runs on clocks generated by the soundcard itself: Both the processor clock for the on-board processor and the synthesized MIDI sample clock. The DSP runs at a multiple of the MIDI sample clock, so the DSP clock is clearly independent of the ISA clock.

So I guess what actually happened is that the firmware got corrupted when it was sent from SSINIT to the SoundScape card. It's the responsibility of the firmware to load a proper program into the DSP so it can generate chorus and reverb, and the DSP will do nothing if it doesn't receive that program. The DSP program, the code that detects the DSP presence and the code to upload the DSP program into the DSP are all part of the firmware (SNDSCAPE.CO0 for your card, likely), and if some of that stuff is corrupted, this might cause the DSP to not be properly initialized. A corrupted sound card firmware might explain the issue with the advanced settings in SSINIT.

That makes sense as there is are several clock crystals on the board including one right next to the socket. It is also interesting as this is the first of many ISA sound cards to have an issue. I'll need to move it to another system that will be a more suitable home.

Thanks for the background info!