Question Kef Q400b has suddenly started to hum! Any advice?

I have a bit of an issue that reared its head last night when I turned everything off to go to bed I noticed my subwoofer (KEF Q400b) was emitting a hum, it has never done this before so whatever is causing it is new. When I unplug the cable from the amp (Denon 6200) the hum stops but when I plug it back it comes back 90% of the time. However a few times I tried (whilst I was fiddling around) it didn't come back (yay I thought its fixed) but as soon as I ran any signal out from the amp to the sub (i.e. a test tone) the hum returned.
I am hoping someone on here may be able to shed some light on this - I am assuming the issue is either the sub itself or the cable. I am hoping that as the hum goes when I unplug the cable that it could be that (but do cables just suddenly fail!) or is it the sub???

Nothing has changed to my gear, I have not added anything and the hum was not there on Saturday after I watched Deadpool. The ONLY thing I did yesterday was tweak/fiddle with some of the speaker setting on the amp - distance mainly as I realised Audessey had got them all way too far away - I have since tried restoring them but it has made no difference and I think that is more a case of me clutching at straws!).

Anyone experienced something like this before - its weird that up until last night everything was absolutely fine.

 

Is it an actual 'hum' or is it a 'howl (like wind howling faintly)? Could be a ground-loop, but could also be the cable. Ground loops often don't stop when you disconnect the cable. So first thing really to try is another RCA cable.