Sounds like a null in the room at the seated position; maybe from floor bounce. Experiment with rugs in between and something on the rear wall behind you.
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?