Depends on you room response. You can get a sound pressure level meter cheap from Radio Shack, set it up on a tripod put in a test cd with warble tones and measure at what frequencies your room amplifies the sound. These would be the ones to tame.
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First of all there are different types of bass traps and they function in entirely different ways. There are resonant types, capacitive types, and absorptive types. The circular ones are absorptive, the diameter and distance from the wall will affect the efficiency of absorption at different frequencies. The height will effect the total Sabiens--or amount absorbed. Wood faced traps are resonant based, they are designed to work at particular freqnencies. Capacitive traps generally have some type of plyable material on the surface. They don't work so much by absorption rather, they re-release the bass energy out of phase, thus their location relative to the speaker is critical. These are generally custom made, but there are a few companies that make capacitive style traps.
The frequencies that should be trapped are entirely dependent on your room. Some rooms require no or very little trapping, som rooms are complete disasters with low frequencies at very high Q factors. In this later, sometimes traps can not even be effectively used and electronic equalization such as the PARC needs to be employed.
The first thing I would do is measure your room to find out what the current response is, then you will know what frequencies need to be dealt with. Our company also manufactures a Test CD which has 1/3 octave test tones that are compensated for by the Radio Shack analog SPL meter. I would highly recommend one of these and the meter--the investment for both of them would be about $50--and may save you hundreds in devices and acoustical treatments in the long run. The CD can be found at www.rivesaudio.com/TestCD2. There's also a listening room section on our site that discuss basic acoustical issues, take a look at the topics on modes, this will help you understand why the room is primarily responsible for those bass bumps and why different rooms have different frequency problems.