Randy,

I am not suggesting that you leave them all plugged into one receptacle permanently. This is simply temporary in order to see if the problem can be alleviated through the use of a single connection. So plug a power bar into a single receptacle and branch off with a few extension cords to see if the buzzing goes away. You have to make sure that the Denon is also plugged into this power bar as you want the subs and the Denon to share the same ground. Don't turn them up too loud either because you may trip the breaker.

It's a pain but this is how you gain experience points \:\) .

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated