use your JBL stadium for the fronts
buy the best sub you can afford - lest you regret it later on.
start auditioning for a good center, if you are going to stick with the JBL stadiums, look for a center that matches them (from the same series)
I'm not familiar with your JBL rears ... can you post a link?
your receiver outclasses your speakers, the majority of the budget should have been spent on the speakers instead of the receiver, but hey, if it's there already ... that's a real good receiver.