Since you are trying to de-couple your speakers from the floor you should really look at getting some Aurio Pro Max bearings (don't bother with the older Aurio 1.2's as they only offer about 30% of what the Maxx's provide) with the optional bearing race on top and optional 5/8" tungsten Carbide ball to go into that cup on top. This will completely decouple your speakers from the floor. I think you would be quite suprised how much your imaging focus improves.
Impressive Turntable project!