Sound is better when I stand up?

I have found a very good tool to use when setting speakers up in a new or existing room is a pneumatic office chair, you can lower, raise and roll it around to check your settings although standing to sitting is not in the cards but once you hone things in tighter it would serve as a useful tool. Enjoy the music

I've gotten better results with an equilateral triangle setup than the .83 ratio Jim Smith recommends. Also best results when tweeters are a little above seated ear height. YMMV.

Agree about the chair. Need one with nothing blocking the sound behind your ears. Also raising up the chair so yours ears are leveled with the tweeters would be ideal.

Reference 3a MM Gen 2. has angled Baffle. No doubt sound is better at height. Put a sharp beam torch/laser light on speaker top(tweeter position) so it directs you, so that ears are matches the tweeter height.

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?

Well @simao we have “solution bukake” (SB) where everyone is throwing out an answer as the solution to the problem…

You probably want a UMIK and REW to do some measurements.

On the “other hand”, your ears hear it, so you need to determine if it is the speakers throwing a high pattern, or a cancelation from the chair, or something else.

Salmon Rushdie’s winged horse in his book “Satanic Verses”, attracted a fatwa against him.
I would bet your winged chair is attack against the ears, but I could be wrong.