With problems which manifest as a difference between the channels I always start at the source and switch left and right connections until I hear the problem reverse at the speaker. The fault is then easily attributed to the component driving the connection which, when switched, caused the problem to move from one channel to the other.
If you get all the way to switching speaker wire with no change then the problem is the speakers.
Also check if changing the preamp volume has any effect. Some potentiometers (even high quality) often have very uneven channel balance when they're set to very low volumes (like less than 1/8 turn). See if the problem exists at all volume levels.
If the problem is traced to the source, preamp or speakers then I'm at a loss to be able to solve it. IF the problem is traced to the monoblocks try switching power cables to see if this helps. However since the hum is inaudible at the listening position I wouldn't worry.
I bet you wish you hadn't listened for it now ... kind of like listening for wierd engine noises in your car just after the warranty expires .... you hear some wierd noises, you worry about them, but there's nothing you can do except turn up the stereo and ignore them ! Best not to listen in the first place. Bit like hifi test discs ... if you like the sound of your system DON'T BUY ONE ... you'll just risk disappointment.