Help with Impedance and Timbre Matching

[Schrute Farms]

I want to buy Left & Right Front Speaker towers and a Center Speaker to go with my new Onkyo 605. If I end up going with a different branded center speaker than the fronts, how do I ensure that the timbre matches?

Also, the Onkyo 605 lists Speaker Impedance as 6-16 ohms. Does that mean my speakers need to fall within than range? The speakers I am looking at have "4 ohms" written on the label.

Here is the Onkyo webpage that shows the specs for each channel: http://onkyousa.com/model.cfm?m=TX-S...s=Receiver&p=s

Thanks! Its funny how I loved my Home Theater setup until I found this forum. Then I suddenly needed to upgrade everything!