Why don't all phono preamps include a rumble filter?

@dwette 
Happy New Year!
when I engage my KAB (in my tape loop), I hear absolutely no degradation in sonics, so, as you say, I guess it is system dependent.  So, what was the root cause of your problem with rumble, and how did you correct it?
Thanks!

"Rumble" is a misnomer since low frequency noise can also be caused by pinch warps or a wavey surface on an LP.  Such defects are exacerbated by a non-ideal tonearm mass/cartridge compliance mismatch.  You can visually see this sort of problem by looking at the woofer cones flutter while a record is playing.  This is a good reason for such a filter which typically engages below 20Hz.

@louisl
Other than getting my turntable properly isolated: I use a Solidsteel WS-5 wall bracket with 3" end-grain maple block. The turntable (Clearaudio Ovation) sits on that. I have changed the OEM feet to a set of Harmonic Resolution Systems (HRS) Nimbus pucks (I have also tried isoAcoustics Gaia and Orea feet...they all work).

As explained in my post above, the real source of rumble was the Clearaudio Magnify tonearm I had...the magnetic bearings were the issue. I changed to a Clearaudio Universal arm with mechanical bearings. Problem solved completely. My KAB sits in a drawer. It isn’t necessary. Fixing the root cause is still the best way to solve problems.

Likewise, Happy New Year to you and everyone else.

@louisl 

I believe the “rumble” you are experiencing is due to sound waves acting on the wall, where it is a moving diaphragm, transmitting the low frequency impulses back to the turntable, which has inadequate isolation. If you can move the speakers outside your listening room as a test, try listening to the same LP on the turntable on the same wall mount location. I believe you will find the “rumble” absent. 

louisl,
You said "Also, isn’t acoustic feedback the same as rumble. If not, what’s the difference?"
I’ve always thought of rumble as something audible or inaudible (pumping) with the volume turned up, the table spinning with the needle down and no music playing, and acoustic feedback as an external excitation by the speakers.