Is a Sub worth the bother?

Agreed based on your system, a smaller sub will match with your monitors more easily than a large woofer. Where I don't agree is the 'cheap' part. A subwoofer is often overlooked as "they all do the same thing" which they don't. The subwoofer has one of the most difficult jobs in any system and 'cheap' subs simply can't do the job well.....well being the key here as a less expensive subwoofer or 'cheap' sub will just pretty much make noise and will overall detract from the great monitor sound. A good sub will add much to a system but a poorly designed sub most often the less expensive models will take away just as much. I would suggest waiting until you can justify the purchase a solid subwoofer and save time and hastle of a less expensive model. Just my humble opinion.

A good sub, Yes. A bad one is better w/o.

Kenobi

I'm enjoying my first sub which is cheap, and in my opinion, fairly good. It's a Polk which was barely over a hundred from bb on a half off sale. However, it seems to underpin my monitors nicely and add something i've definitely been missing. It's not the most articulate thing I've ever heard, but still adds considerably to listening experience.

The kit from Parts Express is tempting. Do they ever go on sale (it's $399, but several of the reviewers mentioned getting it for $300)? There are a couple of subs in the classifieds section here close to my price range one by Elan and one by SVS. Anyone know anything about those brands? I did a little research, but didn't turn much up (especially on the Elan).

A find subwoofers to be very important for speakers that don't go too low. They will often make an appreciable if not substantial difference to the overall presentation of the music with a more solid foundation. I concur with most thoughts here in getting a reasonably quality sub to augment the low frequencies of small bookshelves. You may not blow the roof off with the sub but you will tap you feet along the beat of the bass notes once you get a quality sub in.