Well I have just been schooled. I was only aware of the 4 channel discrete type of Quadraphonics. I'm just getting back into vinyl, and becoming an incredibly incorrigible audiophile more, and more. It started with th AR-15 speakers my uncle gave me and, probably more due to all the SACD's and DVD-Audio's I was into, being a hopeless surround junkie. The sound quality of those formats finally opened my ears up, and I've become rather insatiable.
Back to the question about THX Neural decoding; I am in the market for a new receiver, and have come to take the amazing things it does with a common 2-channel source, let alone anything with encoding...even Dolby encoding, if you try the PLIIx, and then the THX, more often then not the THX leaves you stunned. So in response to the original question, I have unshakable faith that the THX would undoubtedly reproduce the original encoded channels true to their original specs, and then follow through and "fill" the remaining 3.1 channels. I would be surprised if it didn't make it BETTER than the original. Also, for a purist, on the amp I have now, it would be possible to just have the four channels, or 4.1 for a bit more bass.
Hoping that I can find more help on Quadraphonics. Especially before I make my decision on a new amp and/or receiver, and a turntable. I'm assuming that the CD-4 needs a specific turntable, being that there are four discrete channels. Also I would love to get my AR's up and running, but, even if I use a bi-amp method, I cannot find any surround set-up with the massive power handling that they need. It was extremely expensive just to run them in stereo. New to this, but looking forward to meeting a lot of really neat people with the same audio affliction as me:)