Yamaha started long ago catering to the 2 channel crowd with 3 channel add-on amps in order to add HT capability. I bought into it early on with the DSP-E300 (dolby pro-logic), later with a DSP-E492 (dolby digital). The early units were great, but they were really designed to be used with smallish efficient Yamaha speakers that did not sound all that good (low powered amps), so limited usage. Remember that the intent at that time was not the modern 5.1+ discrete sound, but rather stereo focus with a soundfield enhancement.
The dolby digital units were a big move upward allowing full range stereo sound to the rear speakers, and dolby digital level processing.
The added music soundfields are interesting in their intent and execution, but I often ended up selected the pass through for normal/pure stereo sound. Great for wowing visitors though, Church mode with the dialed in reverb would always impress.