Atmasphere's advice reminded me of the old trick with the SME V to reduce effective mass: put the spring-loaded weight scale at zero, and then adjust the counterweight closer to the bearing of the arm. I use an electronic weight to get it exact at 1.72 g.
Based on Atmasphere's advice, I also tried higher loading, going from 475 ohm to 121 ohm (not so easy on the Io, changing a jumper at the back). Result? Musicality: great. "Echoes" sounds a bit sweeter, more mellow. I realize, I am moved by the music - always a good sign. Is it also a bit duller, rolled off, or contracted? Not sure. Maybe a little. I will have to try the middle setting (243 ohm). Testing with the Hifi news record, I did not find any change in the resonance frequency, it is still down at ca 7.5Hz.
More precisely: I can hear the test tone starting to quiver at 8Hz, increasing down to 7Hz, and then I can see the cantilever vibrating badly at 7-6Hz. So the effect of my various measures to raise the resonance seems rather small, and yet, the woofers have mainly stopped pumping. Why? Something else I did? Just changing the position of the player and spindle, a little? Is it the position of the sun (as was suggested)? The collective psyche of Audiogon members? I wish I could report some more hard facts. But whatever the case, I am mainly happy with the outcome. Even with NOS tubes in the first gain stage, the Io's noise level is mostly below the average groove noise level.