Buy High Pass Filter or KAB RF1?

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
  

Oops sorry I forgot to check your question Al. When I get home tonight I will play a record without the sub and will report back!

Thanks for reading my manual!

There was only one time there was audible noise that came from one of my front speakers, but it’s pretty much just noiseless extra flapping of the cones and woofer. 

Ok Al I moved the fronts forward of the turntable and it appears only the subwoofer sitting next to the TT is rumbling now.

When I turned the Sub off there was no rumble present.

Is it safe to assume that the rumble in the Sub is feedback?

I am giving my boy a bath right now but I can try to relocate the sub to see if it all disappears. 
One thing is for sure, I need to buy an outboard preamp so I can get rid of that phono cable extender. It’s horrible. I’m surprised so many people have recommended to just extend it. The hum is so bad it’s not even listenable. Rega seemed adamant that it’s a certain length for a reason. I’m a total believer now.

Update: I also moved the sub farther away and now I have no rumble whatsoever. 
I kept being told to move the table but being limited with existing ICs the way to go for me was to move all of the speakers. 
I’m happy that I won’t need a rumble filter now. 
The question now is whether buying an outboard phono preamp like the Atoll P200SE will make an audible difference because my Marantz AV7704 sounds pretty damn good and I can’t imagine it getting much better. Thoughts? Will the difference really be audible?

Glad to hear you figured out the rumble problem. Having the sub too close will definitely create issues for your turntable.  

I have never tried the phono output in my AV7704, it's possible that it's better than I think it is, but when comparing the phono out in previous pre-pros I've used to a decent outboard phono stage, there is no comparison.  

An outboard phono stage should give you an improvement in sound quality and many of them will give you different loading and gain settings so that you can customize the settings to work best with the cartridge you're using.