Sound is better when I stand up?

It's the tall chair back and wings continuing behind your head - and ears.

Choose a chair that doesn't extend higher than your shoulders so your ears are directly exposed to all room boundaries.

And don't worry about that 8'3" equilateral triangle others are harping on about.  Your present distance of 10' from the speakers is fine if you like the sound and image there.  You could try moving a little closer and see if you prefer.  I sit a bit further away than the distance between the speakers and I prefer the sound staging there.

@simao    

I note you have removed the toe-in.  This is the purist position.  If the production was classic then there was a spaced pair of dipole microphones in front of the musicians.  If you don't toe-in you are hearing what the mics heard.  If you toe-in, the image of the sound will be spread outwards. A 'hole' may appear in the centre.

Speaker toe-in and listening distance are of course related.  Less toe-in is has a similar effect to listening from further away.

I do think you're making a mistake with the wingback.  If you like to rest your head perhaps a chair with a smaller headrest to the back of your head not impeding soundwaves' access to the ears.

Or listen with headphones?