Buy High Pass Filter or KAB RF1?

Is a $25 high pass filter as effective for rumble as the $180 KAB RF1?

I would rather spend the lesser amount. Thanks! 
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.

Chakster, On a purist basis, I would question the use of those AT pneumatic isolators under a loudspeaker, because it seems to me that depending upon weight of speaker and stiffness of the AT feet, the speakers might be free to rock back and forth when driven hard. That results in waste of amplifier energy, as some is lost in order to move the speaker, and distortion, because the signal to the speakers is partially diverted.  I suppose it would be on a case by case basis whether it does anything audibly bad.  Normally, you want the speakers to be anchored rock solid, so all the amplifier energy goes to moving the drivers.
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.

The reason people recommended trying an extension cable is that in most situations it would not cause anything like the hum problem you experienced. I think someone mentioned early in one of your other threads that the Rega’s grounding configuration is unusual, which presumably has contributed to or caused the problem in this case.

In any event, congratulations on finding a solution to the rumble issue. I can’t comment on phono stage possibilities in the price range you are considering, or how they might compare to the one that is built into the AV7704. I note, though, that the Atoll phono stage provides a choice of two load capacitance settings, differing by 100 pf, which is nice although not quite as flexible as the four or so settings some much more expensive phono stages provide.

FYI, BTW, whenever any phono stage provides a load capacitance setting designated as "0," the actual capacitance presented by that setting is not really zero (which would be impossible). My guess is that it may be a few tens of pf’s, and the 100 pf setting is 100 pf higher than whatever it is in the 0 setting.

Continued good luck. Regards,
-- Al