A rumble filter will definitely eliminate woofer flutter. But it will do nothing to improve sound quality. Springs, sand box, etc will definitely improve sound quality, and also in most cases eliminate woofer pumping. You can spend your time and money on improvements, or fixes.
The low frequencies being put out by the speaker are traveling along the floor, into the rack, picked up and amplified by the cartridge and phono pre and sent back out the speakers further exacerbating the issue?
No. That is different. There are two main forms of vibration to deal with, both vibrations but completely different in effect. Your original question is about subsonic vibration. Speakers can't audibly output, you can't hear it, you're concerned with something you can see but not hear.
Then there are vibrations from speakers in the audible frequency range you can hear. This range of vibration will smear detail and color instrumental timbre making individual instruments sound less distinctive. Putting speakers on springs like Nobsound will greatly improve this. As will putting the turntable on springs. They will not eliminate the large amplitude low frequency pumping you are seeing. They will however greatly improve clarity, detail, dynamics, imaging, etc.
This is why I recommend the things I do. Springs, mass, etc work together to solve both problems. Solve. Eliminate. Not patch over with a band-aid. While simultaneously improving sound quality. None of which you get with a filter.
