definitely concentrate on that center channel first. The Q series will just give you a fuller, more diffuse soudn field. Certainly, something is better than nothing, and the POlks will do jsut fine back there. In truth, the surrounds do not HAVE to be matched, like your center and mains (because panning isn't as big of a concern). But, very many people prefer well matched surroudn for music applications, and just because having matched surround is better (in theory) than having non matched surrounds.
Save it for the center.
