Woofer sucking in and out

I can not recommend the KAB unit highly enough. I would not have a vinly based system (the only way I really listen to music...) without one. It's sonically transparent and, in fact, will tighten the bass up a good bit since your amp is having to expend energy modulating your woofers like you're seeing.

The KAB unit does NOT effect phase. Period. Call Kevin at KAB he will be happy to verify and explain this, but here's technical info from his site:

"1. When records are made, frequencies below about 150Hz are combined to mono. This is done to avoid large out of phase groove excursions.
2. When the stereo stylus moves vertically, the two signals produced are equal and 180 degrees out of phase. Therefore, below 140Hz, the music is in phase in the lateral stylus motion, and only out of phase noise is found in the vertical stylus motion. The KAB RF-1 takes advantage of this by electronically combining the left and right channels together below 140Hz. In so doing the filter effectively cancels vertically modulated noise. Vertical rumble comes generally from bearing noise of the disc cutting system. But since the music below 140hz is already mono on the disc, the musical information is unaffected. Below 20Hz, all signals are quickly attenuated at 18dB/Octave."

I have many records that cause this phenomenon with my ported Usher X-718s and it is not an isolation problem (record a bit of the LP to CDR with the speakers off, play back the CDR and see if it still does it, if it does it's not an isolation issue), my cart is perfectly matched (resonance of 9hz). The KAB solves it.

Of course it doesn't have to be done but very often and more often than not it is. It's a common practice that has been around since the dawn of stereo - do a search for "LP mastering 150hz mono" and you'll have enough to read for quite some time on the matter.

Just because you experience this phenomenon doesn't mean there's a problem to address. Period, end of story.