04-26-10: NewbeeBINGO! I have an SL1210 M5G, so I have some experience with this issue. Fortunately, since the Technics arm takes interchangeable headshells, you can change the tonearm's effective mass to match the cartridge compliance by changing to a lighter or heavier headshell. The stock Technics headshell is 7.5g--very light--which works well with high compliance cartridges. I've found that changing to a 12g Sumiko or LP Gear Zupreme headshell raises the effective mass to match well with medium compliance cartridges such as the ones coming from Audio Technica and Denon. I have an AT150MLX and a Denon DL-160 mounted on these heavier shells, and they are both good matches.
No one has mentioned that the probable cause of the woofer pumping is a mis-match between the cartridge and the arm. This will do it every time. It is a arm mass/cartridge compliance issue you should investigate.
According to the charts at the cartridge database, my arm/cartridge resonance comes in at an ideal 10 Hz with this setup on a Technics arm.
Furthermore, proper damping can reduce the amplitude of whatever resonant frequency you wind up with. The KAB fluid damper won't change the resonant frequency, but it will reduce its amplitude, which should make the woofers pump less without rolling off audible bass.