I actually own one of these, and it was coupled with the Alpine flagship CDA-7949 head unit; I own it too. There were several incarnations of this... the PXA-H600. Given that the 7949 had a copper plated chassis, Alpine distributed some of the 600's with a similar copper plated chassis as well. As to the digital part of your question, there are some units that could work with the 600, but they would have to have the processors "protocols" not just the "Toslink" cable access, and unfortunately the 600 has 'input' only for the fibre-optic (toslink) cable, so that can be limiting. (Keep looking though!)
I have seen several 7949's on e-bay, and they come up once in awhile.
By the way, there is a RU-"A" cable/contoller for the 600 (you should have it actually!) it's numbers are: RU"A"-4260. "A" is code for Ai-net. They came with the unit as did the 'p/g/r' cable, which can be tricky actually! And the control unit cable as well as the Mic cable. RCA inputs, and other factors.

BTW, the 7949 has an interface with the cd changer units. If you have a 'toslink' cd changer, you can input it directly to the 600, if you have a non-optical changer, you can connect it to the 7949. (If it is not Ai-net, there is an adaptor!) Good luck!
My advice is to get the Alpine CDA-7949 from e-bay.
Good luck!
e