Buy High Pass Filter or KAB RF1?

@craigert No problem!  In my case I don't think it's a feedback problem, as I recorded (without the speakers on at all) to a digital recorder and played it back and the woofers still pumped. 

I feel you on the passive filter...but for $25 I think it's worth a shot. I ordered them on Friday so got 15% off, which offset the exchange rate a bit.  Still cost me $20 in shipping though. Insanity.

My woofers DON’T flop with the volume turned down. 
It started out of nowhere on my AJA album Black Cow. Something wasn’t right so I switched to Led Zeppelin II and it was rumbling on that too!

The weird thing is why are my front speaker drivers rumbling too when signals below 80 Hz are being sent to my sub?

Craigert, Re this last post, define "rumbling".  Are you hearing a noise or are you watching the drivers do a dance, in and out?

The weird thing is why are my front speaker drivers rumbling too when signals below 80 Hz are being sent to my sub?

Looking at the manual for the AV7704, I see that depending on some menu settings frequencies below 80 Hz may or may not be sent to the main speakers, in addition to being sent to the sub.

Also, I don't think an answer has been provided to a question I stated earlier, namely "does the problem still occur when the sub is not operating? "

Regards,
-- Al