Resonant frequency of the arm/cart has nothing to do with acoustic feedback, so don't let that fool you. Anything from about 200Hz down can cause issues.
You definitely have your hands full with springy floors and a somewhat tight space. I'm not sure how the Vandy's will react, but pulling them out from the back wall another foot might help with the pressure that is building up along that wall. And, if possible, it might be better to pull the table out from the wall as well.
Can you brace the floor from underneath?
