Oakleeman,
I've got the same problem with my Sub 10. Only thing is, this is my 2nd Sub purchased and returned for the exact same problem...moaning sound. I've had both subs for less than 10 days. BRAND NEW.
I have the Yamaha HTR-5940 and Klipsch quintet III and sub 10 (exact same setup as in the demo at BB) and exact same setup on the receiver menus and Sub 10 switches...still moaning.
Moans even when I disconnect the sub cable and leave it plugged in. Ohmed out the cable, all good. Used a different cable, no different.
Did I make a mistake with this? Two subs, same problem, one was returned (I could see the old shipping address to a customer back in Dec 06) and this one, brand new.