Bass Suckout in a Square Room - Remedies and Solut

Hi All,

My problem is I've got major mid-band and lower bass suckout with my speakers.
The bass dissapears in the seating position. But if you stand up either side of my sofa the the bass returns.
I would like to hear from anybody who has a similar problem and what solutions they use to eliminate the problem.
I can't move to another room in the house. But this is a dedicated hi-fi room. So there is a degree of flexibility.
Thanks
Meth