AM Acoustics room mode simulator is a great place to start. Look at the lowest modes and try to keep both your speakers and listening area out of them.
After that, it's good to measure to see exactly what your problem is.
low frequencies
Hey all, Running a pair of Thiel CS2.4 floor standing speakers and the bass is only sounding best when i'm not seated and in a rear corner of the room such as is where the equipment rack is. Is their a tradeoff in other areas of the sound quality to get more low frequency at the seated listening position?