Trust me!, SUB's sound Much Much better on the floor....
I use to have mine on the table, then i moved it onto the floor and it sounded much more solid....
Well, my SUB is shielded i think, check on the box or the manual for magnetically shielded, or hold the sub right at the front of the monitor, move it slowly and check for the effect that looks like you pressed a calculator screen..., you might not feel comfortable doing this though