It depends on the efficiency (sensitivity in db's) of the speakers and the resistance (ohms) of the speakers. If you get speakers that are 89 dbs in sensitivity and 6-8 ohms in resistance your Onkyo-if it is a reasonably powered one, should do fine. There are inexpensive multichannel professional amplifiers made by Behringer, Peavy, Crown, etc. that one could connect to the Onkyo, using the Onkyo as the Preamp surround controller.

Certainly if you are getting lower sensitivity speakers at 4 ohms, it is likely that either a more powerful receiver or a powerful amp is in your future.

Thanks for the input. The speakers should be fine and the Onkyo should be OK, too, as it is not entry level but a few notches up.