Chewy,
Actually, some cds do feature authentic, vintage quad mixes on them. If you played them with the proper decoder, you would get the quad mix. I believe some older Columbia cds had them...like a Santana one I think... But yeah, you gotta have one of them old decoders...
There are also the DTS cds put out by DTS Entertainment. Many classic rock titles with their original quad mixes intact. I'm surprised you've not seen these as they've been out for quite a while. I have a few...like Steve Miller's Fly Like An Eagle, E.C.'s 461 Ocean Blvd...
Playable on most dvd players connected to a surround receiver. (pssst, and they can be copied too!)