Sound is better when I stand up?

Why is this? No matter my listening room (large basement with 7 1/2 foot ceilings or small office with 8’ ceilings), the sound is more open and more spacious when I stand up from my listening chair. When I sit, the sound compresses a bit. Sitting, the tweeters are about 5-6" above my ear level. Should I angle the speakers down?

My chair is at the apex of the .83 ratio Jim Smith suggests for getting better sound. I'm about 3' from the back wall and my standmount speakers are 3' from the front wall. 

What acoustics are responsible for this?