If you are going to reverse the polarity of a JBL cabinet, it must be done AT EACH COMPONENT in the cabinet. You should not simply hook-up the cabinet red-to-black. That will not have the proper effect, because that will result in the signal being pathed through the transducer before the capacitor(s) - this is a DC circuit, and that is backwards. For the crossover network to work properly, the capacitor(s) must see the signal BEFORE the transducer, regardless of cabinet (and transducer) polarity.
To "correct" or re-phase a vintage JBL cabinet, you need to reverse-wire at each component. Simply alter the signal path AFTER the crossover and before the transducer, at each component binding posts.
As example, if your cabinet is out-of-phase (negative), and woofer is wired GRN-to-red post, switch the woofer to GRN/BLK-to-red post. Reverse the wiring for EACH component (this will ensure the mid-range stays 180-degrees out-of-phase in 3-way cabinets). The woofer - and cabinet - will then be positive, and industry standard.
It is handy - but not necessary - to review the network schematics, if you're so inclined, and yours is available:
Link:
http://www.jblproservice.com/pdf/Network%20Schematics/