Thanks for sharing in detail Coot. Hopefully someone will have an idea or two for you. First, let me say that Ohm loudspeakers are certainly capable of presenting a solid and believable center image with wonderful depth and width to boot. Like any speaker, placement is key and Ohms are generally considered forgiving in this regard. The room has perhaps the greatest impact on sound quality and can be especially troublesome for those of us without perfect dedicated listening spaces. Like you, Im one of the unfortunate. Please allow me to share my situation as its similar to yours and will hopefully offer some encouragement.
My typically appointed living room is 13 x 18 x 8. My gear is racked on a Salamander Synergy unit smack in the middle of the speakers and wall. Measuring as you did from the middle of the driver, the Talls are 26 from the front wall, 38 from the side walls, with 80 between them. My sofa being the prime spot puts me at 9.5 from each speaker. While facing the gear, I have a large picture window (with treatment) to my left, a 68 archway to my right, and a 69 archway behind me. The back of the sofa measures 58 to the back wall and archway. The archway behind me opens to a foyer and stairway. The archway to my right opens to another room .. a fairly open design and somewhat problematic.
This arrangement also includes a bookcase, liquor cabinet, chair, tables, lamps, and artwork, etc. There was a lot of experimenting, moving stuff inch by inch until I locked it in. It took a while and it wasnt easy, but the results are great. Its all there .. imaging, depth, and width. Coot, if I can achieve this in my room, Im sure you can achieve the same. That being said, you have whole lot more space adjoining (13,000 cubic ft. - wow!) than I have. However, I encourage you to reevaluate the space and start with speaker placement or perhaps moving your sofa forward a bit. Its surprising how much difference a few inches can make. Perhaps snapping a few photos of your space and emailing John for guidance might be helpful.
Best of luck. Sorry I couldnt be more help to you. Cheers.