Buy High Pass Filter or KAB RF1?

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.

Update: I also moved the sub farther away and now I have no rumble whatsoever.

I’m happy that I won’t need a rumble filter now

Exactly, i don’t understand why anyone need those filters.
You can properly isolate your turntable from vibration, AT’s pneumatic insulators are very effective. Here is how it works.

Thanks guys!
I can’t wait now to try the Atoll. I feel that the Marantz AV7704 is the best bang for the buck for new audiophiles who aren’t rich. Coupled with the right amp it sounds great. It sounds industrial and horrible with my Anthem A5 but beautiful and detailed with my Atoll. I’m sure the ribbon tweeters on my speakers help too.
I am a musician and a drummer, so I definitely have an ear for sound and can tell the difference quite easily.