Best to put the QS8s on the sides and the bookshelfs on the rear. This option can be less expensive than adding QS8s to the rear, however QS8s in the rear also works well. The closer the rear speakers are to your seating the more I would prefer QS8s over bookshelfs goes double if you have multiple rows of seats.

Cheers,
Dean

3M80 2M22 6QS8 2M2 1EP500 Sony BDP-S590 Panny-7000 Onkyo-3007 Carada-134 Xbox Buttkicker AS-EQ1