Hi Jeff
DCX is a cool tool, I think you will enjoy playing with it. On mine, I replaced the XLR digital input with RCA and connected the two lead input wire (from ribbon cable) to the RCA. If there is anything fancy about AES/EBU the DCX has it in name only. The parts quality is low end (amateur) "pro audio." But the DSP aspect is a miracle for the price.
I installed the Jan Didden Active upgrade in my DCX and can recommend it highly. It incorporates a IC 8 channel analog volume control, and IR remote control system. The output stages are removed and replaced with far suprior parts on a totally new PCB. The build was easy and fun. Jan helped me find a mistake I made and I was off and running. The difference in sound is night and day. There are other modifications to get even more out of it, like power supply, clock, and internal wiring for digital input, analog inputs, etc.
Without the mods I think you could make an adapter to bring SpDIF in through the XLR connector. The volume control was the big driver behind the project.
It is designed to be a PA crossover so volume control is not one of its functions. You can adjust gain of each driver, but it is intended to be used with a mixing board. The big knob on the front is for menu navigation, not volume.
Since you;re listening to digital source anyway, no reason to bring audio in by analog, might as well stay digital. The DCX ADC is not audiophile grade.
The DCX does have some kind of daisy chain functionality to link multiple DCX's together, but I have not read about this or what it is used for, whether it is intended for multiple loudspeakers or just stereo loudspeakers with more drivers. Worth finding out..
http://www.linearaudio.nl/ (mod kit details here - but website is down now, I hope it comes back up soon)
http://www.pilghamaudio.com/index.asp?pgid=51 (buy it here)
Rich