Years ago, I experimented extensively with both 6.1- and 7.1-channel surround sound, both with a single surround back channel and with two surround back channels. I was then in a dual-purpose living room space, and the 6.1 with the single surround back channel was most effective, but not enough to totally sell me on the concept. My last two rooms have been dedicated media spaces, and each has been in the neighborhood of 25x16, with a first a 10’ ceiling and now just under 9’. The first house was new construction with a ground-up media room build. It was a big room, and I pre-wired the back wall for 7.1 as a precaution. It turned out I never felt I needed it and when I moved to my current house, a retrofit job, I didn’t give any consideration to 7.1, let alone height and width expansion. Let me speculate on why.