I seem to recall that Deck II is ARTA aware. It certainly is Nubus Digidesign Audiomedia II and 840AV DSP aware. If you want cheap, known good audio DSP speedup on a Nubus machine, I'd go the Audiomedia II. It only has one DSP onboard (56Mhz) but it's a sound card rather than a repurposed video accelerator.

The AMII also has S/PDIF digital coax stereo and 18 bit analogue stereo i/o. The AM 1 doesn't have S/PDIF.