I have my lathe and mill mounted on toolboxes too - similar to yours. I found that replacing the casters with hockey pucks made a huge difference. But I see that you have already removed the casters. Is the base of the lathe flat against the floor or does it rock on the floor? If it rocks, some sort of leveling might help.
It is just sitting on the concrete. I don't think it is rocking, I have shims on one corner. I cant really feel any vibration at the bottom either. Those are nice and that is probably the route I should have went.