Yeah, the 663 falls a little short when all channels are driven simultaneously, but in a real world situation, their power output is enough. However their upper end receivers like the RX-V1800, 1900, 3800, etc... meet their power output specs. I know the yamaha units also have preamp outputs so an external amp can always be added later. I'm not sure if the Onkyo receivers have preamp outputs or not.