Sound is better when I stand up?

Coming in late to the discussion, but regarding wanting to understand the wave patterns, if the wingback is made of stiff reflective material like leather, it would act as a high frequency reflector and you would get phase cancelation at high frequencies. If it's curved then it'll act as a concentrator (like a lens or satellite dish or a bowl behind your head).

If it's thick soft plush material, it would be an absorber for mids and highs and possibly even a bass trap. 

Those are my thoughts anyway. Compare to your situation, run some experiments changing the area behind your head with the wing ack, and a sense of what's going on could become apparent. 

@tom_guyette it would be the latter. It's plush and upholstered which means I'm definitely, according to your observations, getting some absorption mids and highs. 

@simao put something hard (remember hardness is a SURFACE characteristic,.does t necessarily mean rigid or even stuff) flat material behind your head in the wingback to see if it changes anything. Even office chair mat material for rolling on rugs would tell you something. 

Decent rule of thumb is if a material makes a click with you tap it with your nail, it's hard. 

tom_guyette it would be the latter. It's plush and upholstered which means I'm definitely, according to your observations, getting some absorption mids and highs

Ah… no.
(He is saying the opposite.)

The sound hits your ears first.
Then it bounces back off the wingback and splashes back adding to the next wave(s) of sound coming directly to the ears from the speaker.

It should sound similar to standing with your back against the wall.

As the chair’s headrest becomes more and more plush and absorptive… it should become more and more like no headrest. And if it was a wood or leather headrest then it gets worse and worse becoming more like standing with the back against the wall

Just try the “back to the wall” and then try the chair.
And try the chair with a sheepskin or blanket on it. Or even a hard plastic cover.

There should be changes… I would think.

But plushness will not do much at all as the frequency gets lower, unless the headrest is like a transparent net. 

Those are my thoughts anyway. Compare to your situation, run some experiments changing the area behind your head with the wing back, and a sense of what's going on could become apparent. 

+1

SIMAO. When you stand, you are changing the distance your ears are from the speakers, (less) the back wall (more) and the ceiling.  Yet sound wise, you like that position better.  Experiment with duplicating each of those new dimensions while sitting... one at a time.  The .83 thing is a starting point only.  It's not sacrosanct. Your room and speakers change everything so suggest you experiment freely from that beginning. Angling speakers down however, is seldom a good idea for well designed transducers.  Better to raise your seating position. 

Have fun and enjoy the music.