USAFRet :
Put a water glass on the table next to the PC. Turn the audio up to your usual max. Do you see the water vibrating?
No? All is good.
Yes? That's what is happening inside the PC.
It will probably be OK, but why introduce extra vibration?
Sorry, I should have specified. I'm more interested in magnetism in the setup. I'm wondering if the magnet from the sub could affect my mechanical hard drive.
Vibrations aren't too bad as I don't listen that loudly (third floor apartment). I'm more interested in what effects the sub could have just passively sitting there.
To answer your question about why I'm doing this:
My fiancee and I moved. The space for our computers is smaller, so I had to give up my corner desk and get a smaller one. Hers is right next to mine on the left, but she can't afford to go to the left any more. Putting the sub on the left side of the desk would require that she push her desk down more. I prefer using the end table so my tower isn't on the floor anyway, so I thought I could save more space by placing the sub under it. The sub is a bit long, so it would jut out quite a bit due to that black support bar on the bottom of the desk if I put it directly under.