Do we know for a fact they are Magnaflow mufflers? Regardless, any straight through muffler design, that uses perforated tubing with packing and/or media around the perforated tubing (Borla, Magnaflow, etc.), will get louder with age but performance will remain the same. The situation is worse on a Pantera than on most other cars due to the small size of the mufflers and their close proximity to the engine. The only mufflers that don't get louder with age are either chambered mufflers with no sound deadening packing or straight through/perforated tubing mufflers that use a packing material that doesn't deteriorate (ceramic pellets, etc). Those mufflers simply start out loud when they're brand new. Some racing mufflers are made to be disassembled, so the packing can be replaced (Burns, etc) but I don't think you'll find them in a size suitable for a Pantera, unless you have 180 degree headers.