@gnaudio I’ve had several amps over the years plugged in the same room in the same AC receptable. A 300B stereo power amp didn’t hum. My current solid-state monoblocks don’t hum. But a popular US-based, US-manufactured, relatively inexpensive, tube integrated did hum, not a lot, from *both* the transformers and through the speakers, enough that when combined the hum could be heard from listening chair when music fell silent.
I lifted the ground. I ran an extension cord from the other floor of the house. I turned off every breaker in the whole house save for the one receptable being used. I tried ground loop eliminators, like the Hum X. I tried DC blockers, like the HumDinger from AVA. I followed the "how to eliminate hum" instructions from the PS Audio webpage. I tried isolation transformers (Furman pro). I tried a PS Audio unit. I tried a variety of other power conditioners. The company blamed it on the sensitivity of my speakers. I tried different speakers, with same result. Eventually I gave up.
It’s not any kind of discovery that the quality of manufacture of transformers used in different amps varies a good deal (designing to a price point + variable QC), but this demonstrates clearly that what works in location A will not necessarily work in location B, and that makes/models that achieve considerable popularity will not work for everybody, always. Live and learn.
Because mild to moderate hum is not fun or glamorous or sexy or exciting, I suspect it's a problem that occurs more often than is reported or discussed.