Hi,
It seems logical to add another pair of QS-8s, since you already have those on the sides. You could use QS-4s if you wanted something a bit smaller; the timbral match would still be excellent.

I'm a big fan of the quadpolar approach, although I'm sure there are some who may prefer a pair of direct radiators for the back channels.

Regards,

Alan Lofft
Axiom Resident Expert

Alan Lofft,
Axiom Resident Expert (Retired)