My first thought was, if you like what you hear when you're sitting comfortably, legs crossed, or stretched out or whatever you do with them, cigar impedimenta easily to hand, and a little of what you fancy in your best crystal within reach, then, relax, enjoy the music, and . . . STOP STANDING UP!
Facetiousness aside, I think the effect is mostly one of difference. You're more used to the sound you hear when you're sitting down and so there is a novelty attached to what you hear standing up. I 've noticed that if I stand and walk toward my speakers it feels as if I'm walking into the music. This is a nice way of experiencing the holographic effect of a system, but I'm a lazy b**** and much prefer sitting comfortably.