AFAIR, it was a stock Motorola DSP. The only that was odd about it was that it was mounted on the motherboard. You could (and I did) get access that sort of DSP through daughterboards before the Cube, but NeXT took the step of saying "this ought to be a part of the computer itself".

I really disagree about it as a direct ancestor of the contemporary plugin API. These have always run on the host CPU, not dedicated DSP chips (excluding of course Digidesign's original ProTools model, but that was DSP farm rather than just a chip).

And the sort of DSP that was being done on the Cube DSP was being done before the Cube too.

There were many things going on at the time. Digidesign made the Sound Accelerator boards for Mac and Atari in 1988 or so? Roughly contemporary to this. I don't remember the exact year, but I think the first instance of commercial 3rd party plug-ins for a software platform is Waves's dynamics plug-ins for Digi's Sound Designer editor. These ran (non-realtime!) on the 56k DSP on Digi's expansion card for Mac (I don't know if the plug-ins were available for Atari).