Uhm, I'm pretty sure it's bad, but my friend left his Z-5500 sub next to my Case for a while at a LAN, and it was fine.
I have my sub on my right, on the floor, and my case on my left, also on the floor, so.
I'm using the X-540's BTW, and I did the same setup with my X-530's, so just to let you know.