I have an Empeg (Surprise!)
I also have a Pioneer P9 combo (DEX-P9/DEQ-P9), a VDO Multimedia thingamabob (More Linux, YAY!), a PS2 (Possibly even MORE Linux), and a bunch of amps. I will be competing with this setup, so I want it to be as perfect as possible in every way (the competition part is just an excuse, I would want it anyway).

Since the car is `noisy', I'm thinking I'll mod everything to accept digital inputs/outputs. The P9 combo is already digital and the HU has a (one) digital input. I'll definitely be getting the TOSLINK mod for the Empeg, but I'll still need somewhere to plug in the VDO thingie (the PS2 plugs into that).
So... I need a digital input for the Empeg. I do know a guy that's a fairly hardcore electronics guy, but I've been unsuccessful in confirming if the AUX in on the Empeg is converted to digital and back again, or if it's just a straight-through job. In the former case, I suppose it's just a question of locating the AUX-in ADC and bypassing it.

Since the Digital out mod is branded as a `digital sound card', I'm also a little (not too much) worried that there's some additional conversion going on here... Any information on this?

The final and wildest mod would be converting the DEQ-P9 to digital out and adding digital in on the amps. Unfortunately, this won't work since the volume control comes after the DAC. Of course, I might be able to do a second digital cable with volume information... Hmmm... *drool*