It has always been recommended that speakers should be positioned at the 'live' end of the room and the listening position be at the 'dead' end.
My listening room is not unlike yours: 33' long with full width and height patio doors at the speaker end, and a full width, half height window behind me. I have to say I haven't noticed much difference if the curtains behind me are closed or not: will have to do some experimentation and report back.