Warning in manuals may not be evidence, but I think they're still worth mentioning in this context.
That being said I appreciate the information from everyone a lot.
The distance between the top of my sub and the top of that end table is about 6 inches.
I didn't have a real compass to test with, so I tried to use the compass feature on my phone. It seems like within 4 +/- 1 inches from the sub is enough to interfere with my compass. So that gives me an extra inch or so to the bottom of my case, and the hard drive isn't even mounted on the absolute bottom. I think the magnetic field is dropping off quickly enough.
I'll have to test out vibrations and see what those are like next.
Thanks again for the experience and information.