Couldn't agree more. Getting a tower, especially a small one, to do bass is a stretch. Using a sub as mentioned will not only provide a good foundation, but it will alleviate the amp of trying to make bass with a small speaker.
This is going to be very subjective though. If you don't spend enough on a good sub, it will ruin the music altogether. I suggest looking at a bunch of options. Listening is going to be key. As much as the ID subs are a great value, I think it's important you listen to various subs with a set of studio 40's.
You can talk to the store of your choice, and tell them what you are after. Taking a few subs home to your room would be best. I would honestly look at decent sealed subs first. Paradigm has some good subs, but lower in the price range they are mostly vented. Energy makes some decent sealed subs around the 1000 mark. If you are convinced the ID subs are your ticket, see if a friend or acquaintance has one in your price range to borrow.
A stereo pair of subs would sound excellent if you can stretch the budget.
Have fun, good luck.