If at all possible, it is preferable to have all speakers timbre matched. Of course in reality sometimes this is not possible. In this case you will need to listen very carefully and decide which ones match you main speakers best. I've never heard the bravos so I couldn't tell you if they sound like polks or not. You are correct that the surrounds (when using a stereo source) are predominately used for ambience. After you make the purchase, be sure to set the speaker levels with a sound meter such as the radio shack SPL meter.