Buy High Pass Filter or KAB RF1?

Your calculation is wrong. Your nagaoka has 6cu dynamic compliance only when it’s measured at 100Hz just like all japanese cartridges.

For your calculation you must use dynamic compliance measured at 10Hz (not at 100Hz), this is a problem if you don’t know.

Convert 6cu @100Hz to 10Hz

6 x 1.7 = 10.2 cu

this is a compliance of your nagaoka at 10Hz , now you can use this data for calculation of tonearm/cartridge resonance. Actually it can be from 10 to 12cu compliance. The range of RF is 8 - 15hz 

@craigert I am having the exact same problem and dilemma about what to buy as you are! Wow...was about to post myself. I ordered some Harrison Labs "FMODs" (20 Hz high pass)..I plan to put one on one channel to start and see if I can hear a difference in sound. I hope they solve my woofer pumping.

My Focal Aria 906's are getting "woofer pumping" when I use my turntable as well.  I've tried EVERYTHING, it's not an isolation problem (verified by moving it all over and trying different experiments...and making a digitial recording from my 'pre-out' into a digital recorder and playing it back, the problem STILL persists!)
@stereo5 any idea what's actually causing the problem? I have the same issue as you can see above in this message :/

Ok thank you so I am in compliance. Plus Rega told me people use this cart with my P3 all the time. 
Will a KAB fix a feedback problem? Just curious. 
I thought rumble could be caused by feedback but I have somebody telling me a filter wont fix feedback.

@craigert I'm sure it will...I had someone come over today with a preamp that has a subsonic filter built in and the issue went away. I can't speak for the KAB, but I may have to buy one someday as well so I hope you let us know how it works out if you do buy one :)

Just build your own and experiment.  $25 filter is just probably a resister and a capacitor.