If you're going to continue to use the Sony receiver I'd stay away from metal tweeters given your price range as most will tend to sound overly bright and/or harsh. I've seen Soliloquy 5.0s available for around $500 used and they do offer a center channel as well, but they need a few feet from the front wall to open up. They are relatively efficient and although detailed are more forgiving given their silk dome tweeter so they may make for a nice match with your receiver, and they're good enough that you can upgrade your other components around them for the future. Failing this you might check out Axiom speakers, specifically their M3ti($275/pr.) fronts and 100(around $200) center channel--you can try them at home for 30 days risk free and they have gotten great reviews. Best of luck.
Tim
Tim