Sound is better when I stand up?

I had a pair of speakers set up such that the tweeters were in line with my ears, yet I noticed similar experience as you had when standing. I found a Stereophile review of my speakers which stated that the tweeters measured better when higher than on-axis. I ended up raising my chair enough to better reveal higher freqs from the tweeter. So your speakers may have similar qualities.

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.