Sound is better when I stand up?

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.

@holmz Ahhhh. Haven’t joined Audiogon and bukake in quite some time. Thanks for the reminder.

So - the culprit was my wingback! The two wings and its low seating profile managed to smear the sound. On the advice from @elliottbnewcombjr I temporarily replaced the wingback with a straightback that also seated higher and it made all the difference -- sounstage opened up; instrumentation much more 3D than before. 

Which sucks as my wingback is really comfortable! I put four sections of 2x4 beneath each leg to get equal with the tweeters, but that helped only so much.

Still, this will be a cheaper solution than almost any audio addition I can think of!

I would love to see a visual map of the soundwaves and how the wingback blocked or did something else to them. 

Switch out the wingback chair with an office or kitchen chair for a test and see what the difference is. Oh and If your speakers are 8’3” apart, your ears should be about 8’3” from the speakers.

All the best.