I would expect that the size of the transformers in your Bryston and Parasound are larger than what's in your Denon. At least mine are!
My A21 used to have a little hum, and I thought nothing of it. Several months ago I upgraded the lighting ballasts in the room to electronic ballasts. The hum was gone, so at least in my case, noise on the line was causing an issue.
Yes, mine was very much affected by one of my humidifier, when it was on high setting, the A21 hum loud enough to be heard in the next room lol.. The Bryston and the AVRs were not affected by the appliances, but the Bryston's 2X600 VA transformers naturally made louder noise than the AVR's that likely were around 600 to 800 VA at the most. People's sensitivities to the 50/60/100/120 Hz hum varies so there won't be consensus on whether such noises are real issues.
Note: The effects of that humidifier on the A21 was evidently due to the harmonics injected, as the unit was driven by some sort of variable speed drive. In fact the noise induced on the amp was of a higher pitch type, not the 60/120 Hz that was the well known magnetostruction effects of typical power transformers.
