I noticed this thread because I always get excited when people talk about room acoustics and the thread somewhat jumped out at me!
Over the years I have made three observations that might fit well into this thread.
1) With many speakers I find a slight tilt back makes a big difference.
2) The floor bounce between the speaker and the listener has a much larger effect than people realize.
3) The distance above the floor has a sweet spot where you get the best imaging and frequency balance.
It would be important to address the floor bounce by finding the correct floor mat to scatter the sound at mid-range frequencies. You simply place it centered directly in front of each speaker, even if you have carpet already, it's not the same.
I spent a whole winter experimenting with different mats and found the following to be the best so far:
http://www.amazon.com/gp/product/B008BW6LSO/ref=oh_aui_detailpage_o07_s00?ie=UTF...Andersen 221 Waterhog Fashion Diamond Polypropylene Fiber Entrance Indoor/Outdoor Floor Mat, SBR Rubber Backing, 3' Length x 2' Width, 3/8" Thick, Dark Brown
Having this essential but simple tweak in place will greatly effect where you perceive the best speaker placement and listening spot. It will also change how you feel about the mid-range of your speakers.