The subwoofer support in the DSP is all digital. The SAA7705H can mix and
filter the left and right channels in various ways and emit the resulting
stereo bit-stream on pin IIS_OUT2. That pin can be used in at least two
modes: digital audio output or second I2S bus---I think this bus is used
only if there is a slave DSP, which the empeg doesn't (or does it?), so
the pin is probably free. The data sheet recommends tying down unused
pins, so there is a good chance that IIS_OUT2 goes to a resistor.

Even assuming there is an accessible track going to this pin, you would
still need to connect it to an external DAC, so there's no way this
could be done with the existing empeg circuits.