So, kids, I took a few minutes to - hopefully - educate myself a little bit on SACDs.
They are basically DVDs, not CDs, with the same gigabyte advantage.
If they are "hybrids" (they have a regular CD layer in addition to the SACD layer), the CD layer will play in a CD player. If they are SACDs only, or in order to hear the SACD layer, they must be played in a DVD or Blu-ray player that recognizes the format, which is called DSD (Direct Stream Digital).
"SACD" does not imply multi-channel sound; it only means Super Audio CD, for (hopefully) much more detailed/extended high fidelity sound. A mono or two-channel stereo recording can take advantage of the format. This explains my finding one piece that only played in stereo. That's how it was recorded. The other piece was recorded in three-channel.
And that's about enough for today's lesson!