I am planning on building a 7.1 home theater using RC 65i for the front and back speakers, RC 60i's for the middle speakers, TL2 center, and either the PSW110 or the 505. My question has to do with Size of the speakers. I do not want to spend money on the larger 8in speakers if they are not needed. The room is 15 by 20. Any insight would be greatly appreciated. I am thinking about powering them with Onkyo txnr616 or 515. Also do I need to insulate the hollow wall bays?