His Onkyo supports DD PLIIz. Me choice would be to leave his current system - but to mount two speakers halfway between the seating and the center channel. This would give you the mating between your fronts and your rears. Basically, when a helicopter flies over, or when a bullet whizzes past, the DD PLIIz fills in between the front and rears. You would have a 7.1 system, but don't stop there. Your Onkyo supports 7.2,,,might as well get another sub.