here is another reply i got
Dead wrong on all counts!
First of all we don't know the sound if your room, because you have not let it speak. Please turn off the EQ, and report.
Next, 150 Hz is far too high to get a good blend with your system from a sub. Next a large woofer cone is much more likely to interact poorly with your room, especially in light of the problems you are experiencing and the room dimensions you gave.
You are not setting your system by public opinion, but on the advice of those with significant knowledge and experience.
I can tell you that with any speaker interface or crossover, working with the natural roll off of drivers tends to give the best result.
A lot of this bad advice is generated by THX, and the nonsense emanating from Lucas Labs. I have put together a wonderful system ignoring pretty much everything they have to say. Another issue I have with the unsatisfactory Hollywood crowd.
Fortunately I had great mentors and was privileged to have known some of the greatest pioneers in audio.
Please let your room speak. I think you will find your speakers are a lot better than you think, if you don't handicap them at the starting gate.
That "Chris chap" needs a lot of re education.