Browse to www.greatlakespantera.com and select Tech Articles. One of their members did a nice article a decade ago on the infamous 2200 rpm exhaust resonance, that shows (pg2) a pair of ANSAS cut open at the start of the mod.
ANSA was sold many years ago and the new owners at the time 'recreated' Pantera mufflers. Thus the OEM and replacement innards are different mostly in the geometry of the perforated pipe connections inside. ANSA is still in business in Italy. And of course the GTS mufflers are also different inside and out. Std. ANSAS had a large steel washer tack-welded across the outlet pipes inside the muffler bodies as a 'final sound baffle'. Time, vibration and several of us owners with lengths of pipe & a hammer busted the washers off for an extra 10 horses. Those mufflers play "Jingle-Bells" at idle unless you finish the job by cutting the tops off & fishing the loose pieces out.
Pantera mufflers have a difficult job- we're trying to route exhaust gasses from 350-up horses at up to 6500 rpms thru a couple of boxes the size of your kid's lunchbox, while maintaining 92 dB sound levels out the back. Without losing too much power. Decent quietness drops about 50 bhp while freer flow radically increases the noise outside and causes unwanted police attention. Radio Shack and copycats market cheap dB meters that police and racetrack owners use daily. Buy one or you're just guessing. Good luck.