Actually, With all the people out there sitting in front of a computer and their 3-piece pc speakers, a sub isn't such a bad idea. try the M-Audio SBX. It's a home studio grade sub. You run your monitors through there and use crossovers and such. Subs are great if you don't want big-ass 8" monitors on your desk. Plus, you can do a 5.1 setup with the sub. How many movies are done in 5.1??? Like all of 'em! Just be sure you don't get carried away with the bass sounds. Cuz you'll end up having a hard time getting over the thump. Mix in 2 then 2.1, this way you are checking yourself for cancellations and wack frequencies. Also, try mixing in mono.
I even audition through a little 3-piece pc speaker system so I know what
all the little twerps up past their bedtime hear after they've just downloaded my tunes. No hard feeling, just gettin' tired

Mork and Mindy live near me. We have the same colored house.