How To Do You Measure the Quality of Your AC Power?

Just one problem with an ungrounded isolation transformer: some manufacturers ground a metal case to the ground pin, and count on it to prevent electrocution in the event of malfunction.

Don’t get me wrong - I use isolation transformers for everything. But unless I built the electronics myself, I just grit my teeth and ground those transformers. To code.

Thank you  Jea. Nice display of patience.

I used a $20 meter and could tell that my voltage was fluctuating quite a bit during the day which was also moving the bias on my power tubes quite a bit.  I ended up trying a PS Audio regenerator and they gave me a 30-day return period if I didn't like it.  I discovered the THD on my line was at times over 4% and the unit brought it down to less than 0.1% while also blocking nearly all DC that might be on the line.  The regenerator substantially improved my sound quality and also massively reduced the noise floor on my SET amp while also providing electrical spike protection.

Here's a screen shot from a few moments ago of the display.  You can see it's taking the incoming 3.8% THD this evening and dropping it to 0.1% while also correcting the voltage to 120V and keeping it there.  My tube bias never wanders at all now and the system sounds fantastic.  I have no SQ fluctuations over a 24 hr period anymore and all my gear is plugged into it.  I'm not a huge fan of PS Audio gear other than their regenerators - this is what they really got right IMO.

https://i.postimg.cc/MKQR7Nqm/Screen1.jpg

The quality of the mains sine wave does not matter too much as long as it is able to magnetically excite the transformers of your electronics.  Also noise, including RF noise is usually very well filtered out by good quality power supplies. What remains is the current delivery capability of your circuit when it matters. And that is usually dictated by the quality of the internal wiring of your house. İf you are noticing dimming, that is indicative of too much voltage drop in your wiring. İs your house wired with a lower gauge wiring ?  Or is it an older house with aluminum wires ?  Are there any loose, corroded or bad connections from your panel to the plug ?  İs there any 'coiling' of your wire anywhere, causing inductance which resists current delivery if it becomes significant enuf ?

Dimming lights from an exercise machine???????????

That's serious.  No-one else gets that, although I used to get it when I powered up my old Krell.
Sounds like you've just got bad or old or damaged house wiring.
Get a competent electrician to re-wire the house.  Put the hi-fi on a dedicated circuit back to the company fuse with 20 amp wire.
Then enjoy the music!