Don't even bother trying to hook up another sub to your Denon when the 600 is running. Remember the thread about the 600's low input impedance? The Denon can't supply dual subs because of that issue.

Do you have a wall-to-wall distance of 10 feet somewhere? Take it out and that will solve your 50Hz problem \:D .

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