07-04-12: Tobes
Adding mass to the cartridge/tonearm will lower the resonant frequency. Note that warp induced signals are in the 0.5 - 7Hz region, so you don't want to drop the cartridge resonant frequency too low if your cartridge/arm/table already has a problem in this area.
FWIW, I can't recall any cartridges I've used that have fallen outside the broad ideals for cartridge resonance, say 8-14Hz, when a test record is used. However there have been large differences in the amplitude behavior at resonance - no doubt due to the different construction methods of the particular cartridges and arms. ...
Right, but 10-11 Hz is considered more ideal than 13-14 Hz or 5-8 Hz. Adding 4-5 g of effective mass to the OP's rig will lower the resonant frequency from 13-14 Hz to a more ideal 10. However, as you mention, there's the resonant frequency and then there's the amplitude of that frequency. The OP noticed the increased pumping after changing from the stock cartridge on the Xpression (Sumiko Pearl) to the Ortofon, so it seems that there's something in the cartridge swap that increased the pumping. The Pearl weighs 2g more than the Ortofon and is a little more compliant as well at 15 vs. the Ortofon's 13. I don't know enough about the Turbo's housing to form an opinion on its damping, but when you combine a very light arm (8.7 eff mass) with light cartridge (4.1g) and stiffer compliance, you're raising the resonant frequency, and with the lower mass the amplitude would be higher too.
Fortunately the OP's tonearm has a fluid damper, so he may be able to add fluid to lower the amplitude of the resonance. Increasing the effective mass with headshell weights should not only lower both the resonant frequency and amplitude as well.Wrapping the tonearm in PFTE pipe thread tape might help a bit too, both in damping and effective mass. It sure made my Technics tonearm sound better and more linear.