Anyone wish they had a rumble subsonic filter?

A good rumble filter will attenuate differential signal, which is vertical groove modulation, without affecting horizontal modulation. If you have a single subwoofer (or two) driven in mono (mixed Left and Right) this will solve your problem. I bet you could Google "rumble filter" and find a diy kit.
By the way, the pumping of your woofer cones does affect the sound although you may be used to it and not notice.

240zracer...The 18 Hz rumble filter attenuated everything. The mix-to-mono approach can use a much higher frequency, although 180 Hz does sound a bit too high. If you don't use a subwoofer you will need to buy a line level device. But you don't need to buy a whole new preamp.

240zracer...Take Hdm's advice, and get the rumble filter.
Your concern about maintaining stereo separation down to 18 Hz is not reasonable.
1. LP cutting practice (usually) is to mix to mono at low frequency.
2. Many people don't think that LF sound can be localized, hence the popularity of single subwoofers.
3. Although the cut-in frequency of the filter may seem a bit high, remember that its action is gradual, and considerable separation will be maintained well below the stated frequency. Thr filter designers were free to pick any frequency, and they built what worked best.
4. If you don't like it you can easily unplug it.

240zracer...Did you price that 700mH inductor as an air core unit? And you need two. That ought to make your decision easy!