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?  
128x128lourdes
What happens to the high frequencies when you stand up? If they don't change maybe you could consider raising your seating position instead of the heavy speakers.
It’s reflection : you have to use. Bass-traps and/or diffusers. : you resolve the problem. Your music will only sound better, but also more 3-D. Also a better ratio : high-mid-low.