Slim Speakers.

The KEF thin speakers are nice... But you do pay a premium for them.

If the budget is $300/each I'd say you might want to consider some of the Axiom on-walls. They aren't super thin, but they are excellent quality. The M2 are a good example and 3 of them will set you back about $500.
 

Two subs vs one

Need tips on blocking skylight in theater room