Subwoofer positioning

Not sure where to post this, so forgive me if it's in the wrong place.

I've never owned a subwoofer in my life. I still don't really pine for one even now, but I want to buy some speakers for my iMac. The safest and most available, if not popular, seem to be the JBL Creature 2s. I was wondering, if I get these, if positioning of the subwoofer is that important. Basically, it will have to go behind my computer on a shelf about 8 inches below the level of my desk. My desk has shelving each side and above my computer as well. Is the subwoofer sound going to be any good, or should I go for something else?

I'd appreciate any help from those with experience.

Thanks,
James