As many have already advised, you are most likely sitting in na bass null. I had the opposite issue where I had a bass node in my seating position and it would disappear when I stood up. I used an online room mode calculator and voila, I had a vertical bass node. Suggest you try the same. Wont cost you anything. Just get your room dimensions and put them in to a room mode calculator and it will show you what's happening. In my experience, you cannot get rid of the null unless you change your seating position. Or like someone else also suggested, get 2-3 Subs around your room, which will help improve the situation.
Why does bass have more “punch” when I stand up vs. sitting?
I have Rockport Avior speakers and I notice when I stand up the bass is slightly louder and has a little more punch than when I am sitting. This is with the same distance from the speakers. And the speakers are level with the floor. I do like the sound more when standing. Has anyone experienced this? Suggestions as to how to have the bass response while sitting as I do when standing?