I experienced the exact same anomaly but in my in my room it was drastic. Sitting the bass would just go away. To fix it I just moved to a new house not because of the bass but it didn't hurt that my new location sounds better.
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?