Essential tips for system setup

Unsound, you are correct but it is the midline of the back of the cabinet that is congruent with the midline of the tweeter and the woofer. I could also use the front of the cabinet. Also, I have the speakers as perfectly leveled as possible so that at least they are midline parallel to one another, even if the floor is not perfectly level.

Steve , Sorry to say it didn't work in my room.That's not to say it doesn't work.I think my new sofa and chair are the culprits.Re-arranging the room is now in order.

Steve, I still think that all other things equal(!) having the speakers equidistant from the sidewalls helps in areas outside of the considerations at hand. Just a hint with your Thiels, the tweeters are almost exactly inline with the center of the woofers and the woofers centers are a bit aways from the cabinet backs. Take the grills off and use the tweeters to measure.

Stevecham , Interesting.I just purchased a new sofa and chair for my listening/living room.The sofa is both longer and wider then the one it replaced.It also altered and not in a good way the exceptional sound I was previously experiencing.
I will try your suggestion and let you know the outcome

I think I have found a great way to at least get your speakers positioned from a side to side perspective. My method is based on prime numbers and there is nothing really unique to this. But it seems to work quite well and from a physics perspective makes sense; try it.

Take the width of your room in inches and divide this number by a variety of prime numbers like 5, 7, 11, 13. These numbers can then be used calculate quotients that can then be used for multiples of distances from the side walls to place the center of the drivers (in a vertical arrayed speaker system) from the side walls. The premise is that prime numbers don't have resonant frequency multiples except for themselves and 1 and shouldn't promote side to side to side resonant frequencies at such distance intervals. The best thing about this method is that you don't have to have the speakers equidistant from the side walls, just as long as they themselves are centered at one of the distance-divided by prime number quotient points. Multiples of these quotients are also acceptable.

Example: say your room is 148.5 inches wide like mine is. 148.5/5=29.7
148.5/7=21.21
148.5/11=13.5 and 13.5 x 2= 27.0
148.5/13=11.42 and 11.42 X 2 =22.84 and 11.42 X 3 34.26

Thus you could place either speaker at any of the these quotient distances, 11.42, 13.5, 21.21, 22.84, 27.0, 29.7 or 34.26 inches from either side wall are all available.

My Thiels are now (for the left speaker) 27.0 inches from the left side wall and (for the right speaker) 29.7 inches from the right side wall. Again the distances are to the center of the drivers (in my case I used the center back of the cabinet because the drivers are centered vertically in the cabinet) and the reason I chose these distances is to visually align them to a fireplace insert not quite centered in my living space due to an architectural constraint.

The results are that the speakers acoustically disappeared and the soundstage is even, high and wide without any one frequency appearing to be dominant. With my meter and warble tones this is the best evenness of frequency respone I have seen yet. Also, once these distances are in place it is easy to adjust the distance front to back relative to the front wall. I use masking tape on the floor to mark various positions etc.

I guarantee this will work and at the very least provides a great starting point!
Steve