Resonance problem with tonearm/cartridge

I've recently changed my preamp and power amp from a Mac C-28 and MC-2205 to an ARC SP-8 and CJ MV-45. While I'm thrilled with the change in performance and the added warmth and detail, I've created a new problem.

I have "The New AR Turntable" with a Sumiko Preimier MMT tonearm and a Grado Referance Platinum cartridge. While this combo would appear on paper as a great match for a modest price, I'm having a severe resonance problem in the 5 to 10hz range with the tonearm/cartridge match. This problem existed with the Mac gear also, but to a MUCH lesser degree. I would guess due to the LF rolloff in the Mac equipment versus the response to DC of my new stuff. Isolation of turntable and speakers has been ruled out by running a very long wire to the speaker in another room. Still tremendous LF cone excursion.

While I'm aware that a good subsonic filter would help greatly, I would juct as soon understand and correct the problem at the source. My understanding of physics tells me that the effective mass is too low, and to add weight at the cartridge end. Am I barking up the wrong tree?

Any thoughts would be greatly appreciated.

I would first check the impedance matchup between your cartridge and your phono section. There are causes for LF problems other than tonearm/cartridge. Check the owners manuals for both cartridge and preamp/phono section. The specs tables should have this info. Make sure that not only impedance matchup is good, but also sensitivity specs. If these are alright,then look at the other stuff.
The frequencies you specify as the problem are right in the rumble range, so check your turntable setup. Is the correct amount of oil in the bearing? Are all the critical screws tight? Is the motor isolated? Do changes in VTA affect the problem? Also, re-check the suspension adjustment as floor borne vibrations can affect things in the other room. These are just some things to check on. They might not have anything to do with the problem, if you have correctly diagnosed it as a mass/resonance problem. As I am sure you already know, there are tonearm wraps and such for that.
The only way to correct this is to lighten the mass of the arm / cartridge combo to raise the resonance of the two. Otherwise, you'll have to get into other much more involved ways of damping the resonance. You might want to contact Kevin at KAB ElectroAcoustics or George Merrill at Underground Sound. Both are experts in TT set-up and can offer both advice and products to help resolve the situation.

If your problem is based on record warps, there is nothing short of a sharp high pass filter that will solve that situation. Sean
Some years ago, I had a Sumiko FT-3K tonearm, which is very similar to the Premier MMT, and I also had a problem with resonance with using a Grado cartridge (the 8MR at the time). I eventually went back to my old reliable Shure V15V, and the problem was cured. As Sean notes above, you have a compliance problem due to a less-than-optimal match between the tonearm resonance and the cartridge compliance. You can either swap cartridges, or follow Sean's advice (which, as always, it right on the money).
The Grados are known for being very susceptable to picking up hum from the turntable motor due to their high output. If the hum is louder as the tonearm moves closer to the motor then that could be your problem. This is always a problem when using Grados with Rega TT's. If that be the case, you can shield the cartridge from the motor buy placing a sheet of Mu metal under the platter. And no, it doesn't come from cows.