I can't cut stainless, but I think steel will be fine. You'll need some tolerance to allow slight misalignment of the hole centers with the rack. If the holes are 5/8" then 1/2 hex nuts will just fit if the holes are perfectly aligned with the rack. It will difficult to make the floor holes larger, so maybe a threaded sleeve would give more clearance. I'll look around a little. How close is the alignment?
The other idea I mentioned up top was to make a thick metal filler rod for the hole, which will transfer weight from the stock rack spikes through the floor to the 4x4. I can make that from solid aluminum on my lathe. I think I have the metal for that. Then alignment is not a factor.
Wood or metal will move, but that doesn't really matter as long as it makes good contact with rack and floor below. The only concern I have with putting the 4x4 up against the floorboards instead of under the floor joists is that when the wood expands it will push the floorboards away from the joist, which will create severe floor squeaks.
If you want total isolation from the floor, then the columns are not touching the floor or joists at all, and the spikes don't touch the floor either. How do you hold the tall columns vertical without any lateral movement which would allow the spikes to touch the floorboards? rollo's idea would work well, but you'd have to decorate the structure.
What mfsoa did was brace the bottom of the floor joists from below with beams and columns, which stabilizes the whole floor, preventing vibration and supporting the speakers. Then the column can be slightly long to lift the floor up 1/8" to make sure the column is always loaded even when it shrinks in winter. In summer it will just lift the floor slightly. If the floor and joists under the rack is supported by column, it will stop footfalls even if the rack is not isolated from the floor because the loaded columns will prevent any downward movement of the floor. If the floor is stressed upward by the column, then it won't bounce up higher either. It's locked by the column pushing upward.
Check out these speaker isolators:
http://www.sonicdesign.se/sdfeet.htmlHe doesn't ship to US. Maybe shep can do a group buy for us?
