Abstract, pls excuse my misunderstanding, but is this 40 inches from planer to back wall? Regarding rules of 3rds, I had active panels (Audioexklusiv 3A) placed 2/3 from side walls, 3/3 between them, 3/3 or 4/3 from panel to back wall (depending upon listening position, 4/3 for further away). This is panel centre-to-centre.
I got excellent soundstaging without toe-in (as you, I used female vocals).
I got excellent soundstaging without toe-in (as you, I used female vocals).