Subsonics with Grado & Rega Arm

Can you be more specific about what you mean by "sub-sonics"? Are you hearing turntable rumble? acoustic feedback? too much very deep bass? And, by the way, what turntable do you have, and how is the turntable mounted?

I also have an analog front end that uses a Rega RB-900 arm and a Grado Reference cartridge, and have had no problems with this system once I got the turntable absolutely level. In the early stages of use, I was getting a bit of the "Grado dance", but two actions corrected the problem:
1. I made sure the turntable / platter was dead level (this essentially eliminated the "Grado dance");
2. I replaced the spring suspension in my VPI HW-19 Mk4 with sorbothane pucks which eliminated vibration feedback from the floor.
(I'm assuming that you have verified that the stylus tracking force is correct.)

There is a remote possibility that the suspension in your cartridge may have been damaged, but the only way to assess that is to send the cartridge back to Grado for a check-up.

The revised description you've provided is a bit different problem than your original post suggested. Woofer-flexing due to infrasonic signals can, as you imply, be caused by warped records, and may also be caused by a cartridge/tonearm mismatch that causes the tonearm resonance to fall below 8-9 Hz. Since neither of these seem to be the cause of your problem, I'm inclined to think that you have some sort of low-frequency feedback loop somewhere in the amplification chain, but that's outside my technical expertise. You might try dropping a note to our resident tech guru, Sean, and see what he says. You might also try replacing the spring suspension in your turntable with the same kind of Sorbothane pucks that cured the acoustic feedback problems I experienced with VPI turntable.

I got around to looking at your post again, and a detail that I missed in your comments jumped out at me. You state that your turntable is on a cement floor, on top of cement blocks. It is possible that this location is exposing the turntable to a lot of airborne low frequency acoustic resonance, particularly if the turntable is also located close to a wall. Intersections where the floor and wall(s) or ceiling and wall(s) intersect create a lot of bass reinforcement, which is why subwoofers are often placed in the corners of rooms (where there are 3 intersecting surfaces).

You might try mounting your turntable on either a good isolation rack, or a turntable shelf mounted to the wall, and see if that helps to reduce the infrasonic overload.