I used various add on rear channel systems years ago, which did not involve any processing to the stereo channels, but ran from an additional set of outputs on my front channel preamp to a separate set-up consisting of a small processor which enabled me to vary delay, and a modest rear channel amp and speakers. At the time, I was using Quad ESLs with a subwoofer and ribbons in the front channels. The results varied, depending on the recording; in some cases the dimensionality of the added channels was wonderful. I haven't pursued this in years, although I have, on occasion, played CDs or concert DVDs, including Blu Rays over my big HT system. That delivered rock and roll great, but the HT system is nowhere near the quality of the hi-fi system. (At the time, it was a Meridian processor, a bunch of ARC tube amps driving Snells and a pair of Velodyne subwoofers).
Didn't Dynavector make a product that was meant to add channels to hi-fi? I remember reading an old review of it, never saw or heard one in person.