I'm doing this right now, although I have a VP100 on top of my sub. If you look at some
pics I posted, you can see I put a bit of no-slip mat between the two (look in the third last pic). Just trim it so it hides between the speakers (last pic) and it should be nearly invisible and stop it from sliding around or vibrating off. You should be able to pick it up at any big convenience store (I believe). I haven't compared with/without it for sound differences because I don't want to scratch up my sub, but as long as you place the speaker near the front of the sub (to avoid sound reflections), there's no reason it would change the sound at all (as far as I know). If you move your speakers later and the mat leaves a little bit of residue (like mine does), just wipe it off with a microfibre cloth. Works great and doesn't scratch at all.
