Check out these Epiks, VERY nice subwoofs for not much money...
For starters keep speakers out of the corners, and away from the back wall- these positions tend to reinforce bass, especially if the speakers have rear-firing ports. I have a similar sized room (11x13) speakers on the long wall. Speakers are just over 5 feet apart and 18 inches from the back wall, toe-in is one inch making them 19" away from the back wall on the outer corner. Listening position is about 6 1/2 feet back from the leading edge of the baffles. My speakers are pretty wide so yours may need to be further apart if they're tower type speakers. Placement can make mediocre speakers sound better than you'd think, and great speakers poorly placed will sound terrible. Experiment-I put blue masking tape on the floor when I find a placement I like so If I make a change it can be reversed if it isn't an improvement.
Hopefully, through positioning, you can reduce the problem to a narrow range of bass frequencies (ie. 50Hz-->60Hz) If it still bothers you, it might be necessary to change out the speakers for ones that roll off below about 60 Hz or use a parametric equalizer or low pass filter.
You may need a sound level meter, a tripod and some test tones (on a CD or otherwise) to fix these issues.
The suggestion for sealed box speakers is a good one, in my experience. I've got a room of similar size, and had good luck with a pair of little Spendors (forget the model), and I'm back to using my beloved Celestion SL-700s. It took about eight months, but I finally got 'em positioned just right. (Long wall, about a foot and a half from the rear of the speaker to the wall.) I tried several ported designs, but couldn't get any of them to work in that room.
I have lived with a 16x12 room for ~20 yrs. now and have tried dozens of different speaker positions; and what has worked best for me is an "asymmetrical placement". Meaning the speakers aren't placed on the long or short wall but instead I had them firing across the diagonals of the room. With MUCH trial and error I was able to find spots where the bass didn't boom (and obscure detail) and I could have a wide/deep sound stage with palpable/dimensional images populating that space.
Eventually I found several "G" spots of the room.
Have tried it successfully at other friends systems so it's not just a my room thing. The biggest hurdle for most people seems to be that it looks funny and/or their significant other wouldn't go for it.
Let me know if anyone out there tries this and agrees it has merits.
Phasecorrect , there is never a straight answer. No two rooms are the same.
Since bass waves can easily travel through walls, some of the interference
patterns in your audio room may result due to waves reflected by walls of the
To determine the best place for the speakers, i.e. that excites the minimum
amount of room modes, I recommend to place the speakers agains each of
the walls. Of course, the position of the listen chair is also critical, i.e. make sure
you are not siting in spots where you have destructive (no bass) or constructive
(too much bass) interferences .
If you have the possibility to do this, you would be surprised at how significantly
different the bass is when going from on wall to another.
Many ported floorstanders these days include foam plugs for the ports for tuning the bass to the room. You might see if you can get some foam that will fit in your port(s) and see if that helps. Sometimes I place a rolled up terry washcloth (dry, of course) or hand towel in the port tube to attenuate the port output and increase bass damping.
PSB and Monitor Audio are two makers who include the foam plugs and have both front- and rear-firing ports. With two plugs and two ports you have four possible configurations.
If you still have the boxes your speakers came in, wad up newspaper and stuff it into the two speaker boxes and place them in the corners behind the speakers. If it improves bass clarity, then you know real bass traps will help. Or you could just wrap the cardboard boxes in neutral cloth and use those instead.