I agree with dlcockrum above - Finding the best placement positions for your speakers and listening position is the no-brainer place to start. It is by far the least expensive solution suggested and may have the greatest benefit. If this doesn't fully solve your problem, it should at least help it and make it that much easier to solve with one of the other suggestions above.
If you are indeed sitting in a null, a sub isn't really going to help you, just compound the problem. I was amazed in my own system using the meter how I could make my music sound so much better just by moving speakers and listening position a matter of inches. There are many test CD's sold for this specific purpose and they have simple instructions on how to test your room. You might actually solve your problem in 5 or 10 minutes, or at least go a long way towards doing so. Good luck.
If you are indeed sitting in a null, a sub isn't really going to help you, just compound the problem. I was amazed in my own system using the meter how I could make my music sound so much better just by moving speakers and listening position a matter of inches. There are many test CD's sold for this specific purpose and they have simple instructions on how to test your room. You might actually solve your problem in 5 or 10 minutes, or at least go a long way towards doing so. Good luck.