If you have a 7.1-speaker system with your Onkyo TX-SR606, assuming that your room is big enough, go for 7 speakers (4 surrounds).
* Also, why not get all of them from PSB, that way they'll be better 'timbre match'.
Because they are still in their box (the Klipsch), I will return them,
and get two pairs of PSB speakers for your four surrounds.
They don't have to be big, as your subwoofer will take care of the lows.
PSB makes some nice speakers and are good value too (reasonably priced).
One with a 5" woofer should do it. Just cross them at 80hz, or 100hz if they sound better that way.
You do have a sub, do you?