Actually, the vast majority of subs that come with PC speakers are NOT magnetically shielded either - only the satellite speakers are. They figure you're not going to be putting your sub next to the PC. That being said, they're probably still safer than car or home audio speakers because of their generally lower power.
If you put your PC on one side inside the cab and the sub on the other, that ought to be enough distance to not have to worry about anything.
I did once crash a hard drive because it was too close to a home stereo speaker, though. Lost my entire MP3 collection.