Oabeieo is pretty knowledge when it come to all pass filters so he is probably onto something with his method. I have always resorted to using upmixers, so I too would like to know specific ways/methods of manipulating All Pass filters for 2 seat tuning (even if it only worked for a specific setup).

I do remember subterFUSE mentioning that measurement software like Smaart or SysTune is usually required to be able to get All Pass Filters correct. But again HOW people are doing it in their build is the big question.