Do you use any preset equilization (rock, jazz, classical) or custom. Check the equilizer on the iPod, turn it off, if you can't turn it off set it flat like a straight line right down the middle.

Also check the volume on the iPod itself which could be a factor in having to crank the volume on the Denon. Set the iPod volume to 90% see if that helps at all.

Has the humming gone away when changing outlets?

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne