Always get the center that matches your front left and right speakers, or else you will have a very schizophrenic soundstage. Not only that, but a crap center will mess up the whole sound, so it won't matter how good the left and rights are, they won't overcome the sound of a bad center speaker. The corresponding center for the KEF Q300s is the
Q600c, and it is a lot more than $180. If that is too expensive, I would use a phantom center, which is where the avr blends the center channel programming into the left and right fronts in such a way that it sounds like you had a center speaker. You could also simply buy a set of speakers which has a more affordable matching center. By the way, the best center channel speaker is identical speaker to your left and right fronts, but I don't think you can buy a single Q300. The only reason they have horizontal center speakers is for its convenient form factor, and also a nice way to overcharge for a speaker.