Lot's of good advice above (I am now inspired to add a second sub!). To relay a little personal experience, for the past couple of years I had ported monitor speakers in my 11'x 22' room. I have my setup on a short wall, which effectively requires that I use a near-field approach. I wrestled with excessive bass "boom" for some time, but after some experimentation I noticed that I could plug the ports on the monitors and control the bass very effectively. Based upon this experience, I have recently decided to go with a sealed monitor (Merlin TSM) and added a Martin Logan Dynamo (10") sealed sub. Like Timrhu, I found the corner didn't work well and I have my sub positioned just as he does in his system pics (sorry my pics are not up to date). I get great bass response with no boom - except occasionally on poorly recorded material. So, the moral of my story, you can get good bass in a small room, but I would highly recommend sealed enclosures both for the speakers and sub. Also, experiment a lot with your crossover and volume levels (get an spl meter) and fight the temptation to turn the volume levels up. The goal, I believe, is not to hear the sub per se, but rather to miss it when you turn it off.
