PSA serves around 60-70 cities from CLT, DCA and PHL. The 700s don't fly into DCA or PHL. DAY and CAK are maintenance bases and TYS is nothing more than a crew base mostly feeding into DCA and PHL. With the 900s coming, PSA will probably open a new maintenance base somewhere which could lead to another crew base. DAY and CAK are not large enough to accommodate enough 900s at one time.