****Throwing weight at the I-beam near the spindle pivot does not increase the arms rigidity.****
Exactly.
BTW, there is a (not particularly elegant) way to experiment with lowering the I-beam compliance without use of the double spring. You can wedge (carefully, of course) a very thin piece of some very rigid material cut to fit that space, into the cavity between the spring and outer end of the counterweight cap/clamp. This will effectively allow LESS horizontal movement of the I-beam. I think you will all be very surprised at the
difference in sound.
Ain't this fun?