That gave me chills. I have the Wharfedales too (8.2 model) and have the hum problem on ONE of them. Only one. And when I switch channels, if it was the feed, logic says the hum would follow the channel, right? Wrong. The hum stays in the speaker on the right side of the room.

HOWEVER.....

I had reason to go behind the knee wall (to tack up some fallen insulation) and discovered a junction box there I never noticed before. I have my electrician coming in to look at that, but with all the other testing I did, there HAS to be some spurious AC causing the hum because the speaker sits right in front of it.

I also took both speakers into another room, with first the same mixer and then a second mixer, and there was no hum. So as the responders have suggested, it seems like an AC related issue. If you swap left and right out of the mixer, does the hum stay with the channel or the speaker?