Micro, how do you think my room will suit or not suit Duettas?
Loft space, so descending eaves 30 degrees from mid point apex max 10' high down to 4' high side walls, 18' room width overall
Overall depth 55'
Flr area 1000 sq ft, volume 7000 cub ft
I have ability to place Duettas 5'-10' from rear walls, 8' apart, at this spacing they end up 2.5' from side walls, and outer top corner 1' from descending eaves
I can sit any distance from them 8'-15', and still have half the room behind me
Micro can't answer that. He is not an astrologer. What you need to do is, Jon right now has a cheap duetta I think. Pay him 500 quid to drive it over and test it. Test it with your NAT and a 500 quid Crown class D 500w amp. If you get the bass, pay him to restore it to a Deutta Battlestar Galactica. Keep the crown amp to burn in the panels for the first 200 hours, after which you can always sell it with minimal loss.
Correction: Duetta Battlesta Galactica
Even if I was an astrologer I would not be able to answer. But Ked advice seems reasonable. Before committing with an expensive custom item you should try a standard version for some time. Your space looks great - did you get an RT60 versus frequency measurement?