Buzzing noise from speakers at high volumes?

I'm using an Onkyo tx sr576 avr connected to my pc using toslink, the speakers are a pair of Jamo c93s.

When I turn the volume up (very high), a loud buzzing sound quickly appears regardless of which source I've selected (digital vs analogue input doesn't make a difference). Oddly enough, when the receiver is trying to play something using dts encoding the buzzing isn't present (although that's not much use given that most stuff I play doesn't use dts).

Also worth mentioning that the only times I put the volume up that high are when watching quiet movies. If I tried those volumes with music, I'd end up deaf very quickly, so it's not noticable most of the time.

Is anyone here familiar with the problem and how to fix it? Looking online the only thing I can find is a bunch of people talking about ground loops, but I thought they wouldn't apply when using an optical connection?

Edit: forgot to mention, the humming sounds suspiciously close to 50hz (mains power here is 230v, 50hz).