Faint buzz in speakers

"simply make sure all lights with dimmers are turned off while listening”

"I noticed that sometimes it's a light rheostat (sorry, dimmer switch) making noise in the circuit. However, it's usually a cheap charger that is the culprit in the circuit. "

- better amp designs are immune to AC power harmonics, introduced by fast switching currents. so far I found Accuphase is the best! 

"it turned out to be proximity to a router. "

- I had similar issues before changing pre/amp to Accuphase.. to identify RF noise immunity issues, just touch/move arm to diff components and cables, around amp/pre/turntable/CD etc at max loudness (TT/CD/etc idle).

OP. From your updated description, there is absolutely nothing wrong with the amp - its just regular residual noise. Forget about it and enjoy the music!

I'll throw my guess into the ring: Filter cap on it's way out. Oh, and don't neatly coil excess cable carrying signal.

PS- Modern dimmers, including smart dimmers are not noisy the way the old one's were, mechanically or electrically.  You certainly can have remote controlled mood lighting in your listening room. 

I had a similar problem a few years ago.  In my case I had a subwoofer and I had to ground the sub plate amp to the preamp.  It cleaned it up