Mechanically, yes, you can easily replace the driver with any other 1" throat screw-on component with similar specs. The threads are the standard 1-3/8" diameter and should screw right onto the existing horn lens with no problem. However, consider that the horn size and shape has a lot to do with the sound of the driver as well. A different driver may very well create a new set of problems for you such as changing the mid-range quality, being too loud for the woofer, or whatever.
I believe the "harshness" you are hearing may be inherent of titanium drivers. The diaphragm dome material is made from a thin layer of titanium. You may want to consider switching to a driver with a different diaphragm material like Polyester or something. BMS comes to mind for making top quality drivers out of Polyester.
Furthermore, any changes you make should be done to both PRX535 cabinets so they will both sound the same.
