Bass is clearer and more impactful when I stand vs. when I sit

Hello, 
I am having this issue with my speakers and my room. When I stand, I found that the bass is significantly clearer, more impactful and cleaner than when I sit. Also there are some bass I didn't hear before, now I can hear when I stand. 

I tried to tilt my speakers a little bit downwards but it doesn't solve the problem. 

What's wrong with my room/speakers and how can I fix this. 

Thank you. 
Huy