Sound is better when I stand up?

+1 jond.  With most speakers your ears should be at tweeter level to sound as spacious as intended.  Have you tried tilting the speakers towards you or raising your listening chair?

I was talking about the electrical phase at different frequencies that are shared around the crossover region between the tweeters and woofers, not one speaker being wired out-of-phase to the amplifier.  It’s a common issue with multi-driver speakers.  

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.