Wondering about adding surround back spkrs.

Hello-I'd like to add a pair of surround back spkrs to my existing home theater and am wondering about tonal compatability. Running M & K K7's all around with a Onkyo Ht-rs106 and Oppo BDP-83. Short of finding another pair of K7's I'm considering a pair of Polk Monitor 30 speakers. I can find a pair extremely cheap on line and would appareciate some feedback as to their sound meshing with the M&K'spkrs(I am not familiar with the "house" sound of Polk). Also, the M&K's are 4ohm and the Polks 8-would this be a concern(and why....)? Thanks!
You must first read Floyd Toole's 'Sound Reproduction' book. Floyd is the authority on speaker placement. Your are already on the right track in that it is the placement that makes the difference.
Not being able to read but having been into surround sound before most knew it existed; that would be mid '80's--- The room size and all the usual criteria apply.If you like what you have--just buy the speakers--i.e. jump in the water. Rear speakers will add so much enjoyment. There are so many pictures of setups as a starter guide. It doesn't have to be perfect right off.
Heck, starting out with lamp cord for those rears will tell you; as in give you a good taste. I'm sure once you are there in the game you'll know where you want to go.