Sounds to me like Jadem6 has it right. Presumably your listening position is far away from a wall, and probably in an anti-node. In my large listening room I get a similar effect. If I move stuff around to fix the bass problem I get poor sound through the mids and treble - and even then I just get more bass, not necessarily better. I fixed the problem by adding two subs, after long and frustrating experimentation.
Where s the bass?
Why does the bass sound so much deeper outside of the sweet-spot???? Even in the next room!!!! I don't believe this a shortcoming of any particular part of my system, but some sort of government conspiracy or something. I don't even care that it's not in the sweetspot, but why so much deeper elsewhere????