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.

^^rb