You should get to keep the volume control as the signal feeding the board I am building is the same one going to the Empeg's internal D/A converter.

Except that the signal they feed to the internal D/A converter is full-range, and a separate command sent to the DSP itself is what controls the output volume of the DSP (as well as other things such as balance, fader, EQ settings, etc.).

So I think he's right, the output board you're making will feed raw digital bitstream to the output, and it will be up to your external digital decoder and amplifier to supply the volume control.