James,
Here's where it is mentioned in the DSP33 handbook on page 30. I couldn't find the equivalent in the DSP500 manual.

There is also this Connecting Sub with MC200, 561 and DSP33s thread from last year in which I asked similar questions about the integration of a sub with DSP speakers.

Maybe of interest.
Cheers
Tim

RoPieee Allo DigiOne Signature, 561, DSP5000, DSP33, SW1600, M3.
HH1 Number 48