Faint buzz in speakers

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

Some amplifiers are noisier than others, efficient speakers will bring this out. However, if you can't hear anything from the listening position I would leave it alone and enjoy your music.