The best way to eliminate room resonances is not to attenuate the signal, but to reposition your speakers. What you need to do if find a 'null' in the resononance, and position your speaker right there. By placing it in the null, the sound from the speaker will not excite that resonance frequency, and you will have eliminated it completely. The biggest improvment I have ever made to my system was moving my speakers 9 inches. And it was the least costly one too.
To read up on speaker placement, go to infinity's or harmon kardon's website, and go to there technology or white paper section. They will have information and tools to help you to determine and eliminate your room's resonances.
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