Speaker hum/crackling?

[Automission]

on all my speakers and on several different audio inputs (DVD, 360, Sky box) I'l notice a very faint humming noise. Sort of crackling. However I only notice this if I set the speakers to 50-60 volume which is basically the maximum volume.
If I leave them set to their average volume of 20-30, I can only hear the sound when I put my ear right up to the speaker. I'd also like to mention this is when no sound is being outputted via the speakers, the DVD player is just switched on, not producing any sound.

Are these signs of blown speakers, as I really don't see how I'd of blown them. Never went above 15 volume during the first hundred hours. I have tweaked the settings so they all output around +5db instead of the -4db or so they were set with. But this is only because when they were set to -5db, I had to crank the volume up to 40, which left little room to go louder.

If anyone wants a full run down of my specs, please say. I'm curently running an Onkyo 606 and the Monitor audio Bronze Package.