That's a very thoughtful post. Since you seem open minded to opinions, while waiting for HD to response, I'll offer my 2 cents.
HD did say: "I suspect part of it is buyer excitement but
glad you're enjoying it. Curious, what volume do you listen and at what distance? "
That's fair comment, because lots of people, including me, have experienced the effect of "initial excitement" and/or expectation bias. Years ago when I added my first 200 W Adcom amp, I thought there was significant improvements. Years later, after several more additions, including class A differential preamps, and some $2-$4K power amps, I have learnt that when driven well below their design limits, preamp/amps don't really sound all that different. I am not saying they sound the same, but not what I would call night and day kind of difference.
Your answers to HD's questions about the volume and distance are revealing. I took a quick look using the peak spl online calculator and found that using those SVS Prime tower sitting 12-13 ft, you do need pretty much all of the available power from the 40 lbs TX-NR809. However, based on the S&V bench test results on the TX-NR807 (predecessor of the 809),and the Audioholics bench test results on the UPA-7, the two units are actually comparable in terms of power output into 4 ohms. Their rated dynamic power rating are also comparable, within 1 dB or 2 at the most, in favor of the UPA I would say.. What the UPA brings to the table are mostly the ability to drive 4 ohm loads, all 7 channels driven ability to higher levels for a longer period of time, allow the AVRs to run cooler, improved reliability, as well as a little more dynamic headroom. I say a little, may be just 1-2 dB, because the Onkyo 809 itself also seem to have very good dynamic rating.
https://www.soundandvision.com/content/onkyo-tx-nr807-av-receiver-ht-labs-measures
https://www.audioholics.com/amplifier-reviews/emotiva-upa-7/upa7-measurements
The question remains, why some heard so much more of a difference than others? In your particular case, one thing that jumps out is the level. It is a well know fact that for a fair comparison, it needs to be matched matched to within as little as 0.5 dB, and in general people would perceive the louder one as better.
There are numerous articles related to the need to level match topics, including Dr. Floyd Toole's, below is one that I have never read before as I rarely read anything written by a "mastering engineer".
http://productionadvice.co.uk/level-matching/
The UPA-7's gain is claimed to be 32dB, Gene apparently didn't think it was that high but still he said in his review: "so its gain is about 3dB hotter than typical" For details, please refer to the audioholics.com review linked above.
That could mean if you had set the volume to 82 without the UPA, you can now set it to 79 and still listen to the same level. That 3 dB difference would otherwise give you the impression that with the UPA in the loop, the sound is fuller, punchier, clearer etc.. That's just in terms of the average effect, for contents that have lots of higher level dynamics, the UPA might get you another dB or 2 advantage, that combine with some expectation bias effect, may just amount to the night and day kind of difference that you are hearing. It's all good, nothing wrong at all, just sit back and enjoy!
