>>>All of my home audio equipment (TV's too) has been on a double-conversion UPS for years now.<<<
Correction, my power hungry power amplifiers are surge protected, but not on the UPS. Only the low wattage consuming front end gear in my audio system is on the UPS. Even some of *that* front end gear is *even more* power conditioned after the double-conversion UPS. My headphone amp (Gustard H10) has a smaller dedicated 500VA balanced AC transformer from which it gets its power from, and my PC tower (yes, I still have one of those) which contains my Audioscience ASI5002 soundcard, also has a dedicated 500VA balanced AC transformer feeding it. I also use a dedicated isolation transformer for my CD players (of which I have two). I have the measuring devices to measure noise floors, and my amps receive a -95 dB SNR signal. In listening terms, of course that is a *very quiet* noise floor. THAT is the figure I pay most close attention to.
Even though in my previous posts I have put a lot of emphasis on THD; maintaining a figure of 1%- 2% it's not always my numero uno priority. Just like many of you, my ears play a bigger part. For many years I've owned a Fluke 43b (like mentioned above), and was obsessively measuring THD on my AC lines. Since readings in my house were always within the 1%-3% range, I sold it, but I still stand behind the neutral theory, because in the electrical world, there's been so much discussion about it.
As I may have mentioned, I was Chief Engineer for numerous radio stations for over 30 years (try interconnecting 13 studios in the same building, which are all interconnected to each other, full of audio gear, and have an end result of no audio humming or buzzing - it's quite the challenge).
One time I took my Fluke 43b, and plugged it into a power strip which had at least 12 pieces of gear plugged into it, at one of the radio stations I was working at. This rack of gear was about 6 feet from the primary power service entrance for the building (lots of EMI I'm sure), directly under a 340' self supporting broadcast tower (lots of RFI). I was amazed(!!) when the Fluke revealed a THD of 0.3% at/on this powerstrip. In all my years of owning that Fluke 43b, I've never ever found a reading that was under 1%, anywhere, but of all places, in a rack full of gear, at a radio facility, directly under a broadcast tower. Now I think this had something to do with the rack being so close to the service point entrance. My theory on this is that the neutral feeding this rack had a very short distance to travel, and had little chance to inductively pick up noise via that short distance, which may be why it had such a low amount of THD. I was very happy to see this reading, as this rack contained the audio processor for the over the air (OTA) audio, and if I'm feeding AC to this audio processor, which has a 0.3% reading, glory be, all of the listeners of this radio station benefit.