I assume that the Pioneer has preamp outputs (that particular receiver is from the higher end of the product line, as I recall, so I would expect that it does), so one option is a stereo amp or pair of monoblocks for the mains and a three-channel or five-channel amp later for center and surrounds (the five-channel allowing for growth to 7.1). The center and surrounds could stay on the receiver for now if you are satisfied with the results you are already getting from the 45TX.