Sorry to take so long to get to this thread. I have been travelling for the last week, so I have not had the time for A-gon (and still don't, but I like this site too much to stay away).
Even though it's pretty easy to figure out what the room response is going to be I would recommend a couple of things. First, measure the actual response. A Radio Shack analog SPL meter and our Test CD (compensated specifically for this meter) is a very inexpensive way of getting objective measurements. Yes, it is the subjective sound that counts in the end, but objective measuremnts let you get to this end much much faster.
The bass is the biggest problem for your room. It is going to boom around 40 to 50 Hz. If you look at the axial mode (that's the simpliest from one parrallel wall to the other mode) you will see quite a build up in this area. Tube traps will not likely have much of an effect unless you buy a lot of them. Yes, they will make a difference, but if you start measuring your room response, you will see they are not doing the job needed because they are not covering enough surface area to really be effective. Even if you did cover the area needed you would find that their Q factor (width of response) was so wide that you would be effecting frequencies that should be left alone. The only passive device that even has a remote chance of dealing with the problem is the RPG Modex mentioned. They are made in 40, 63, and 80 Hz models. You will need to start with about 4 of the 40 Hz models, measure the room and go up from there based on the results. The Q factor has about 50% overlap from one model to the next, so it's pretty neat what can be done with these, but they still are not very specific to the frequency problem.
Your other, and much easier, and definitely will work option is the PARC
. Yes, I'm biased as we manufacture this unit, but it is designed for JUST this purpose--nothing else. It's probably the biggest problem in existing rooms, and the bass modes are so detrimental to the entire soundstage. Most people are not willing to do the construction necessary to change the room mode problem, and the PARC is thus the solution. In my recent travels I met with a very respectable dealer, that most probably know. He is very big in 2 channel and told me "I don't believe in this. I don't think it will work, and I don't think it will sound good. At best it will color the sound--and that's not what I am about." We put the PARC in, and in 30 seconds he said, there is no question. This device works, does not color the sound, and deals with the problem just as you've said. He bought the unit, became the dealer, and was demoing the unit to a customer as we left his store.
In a way, I didn't want to share that story--it sounds like an advertisement. But it illustrates the point--hearing is believing.
There are some other issues in the room that will need to be addressed, not as severe as the bass mode problem, but still important. Some of these have been mentioned, such as first reflection points and taming those. That's a definite and should be done. I won't waste time with what's been said already. The area that has not been addressed is the sweet spot. You plan a home theater in a small room. In most cases the sweet spot for a room that size would be about 1 cubic foot (maybe 2). You can expand that dramatically if you wish to, but it does require significant engineering and design. This is what we do, and can increase the sweet spot to a very large volume (like 1/3 of the room--with uniform sound control). Small rooms are by far the most difficult to deal with, they take more time and more engineering to get them right, but the end result can be dramatic. We recently finished a project in DC that makes a 19 x 13 x 8 sound like a 25 x 18 x 11. The sweet spot is huge, you can walk around the listening chair and have beautiful imaging and sound stage.
Please do use all the resources on our site, and if you have any questions please call us. We are always happy to discuss listening rooms--even if you are not employing our services.