factory flaw in a muffler?

This must be a rarity. Has anyone ever been unfortunate enough to purchase a new muffler(s) that had an internal problem causing a substantial drop in performance? Im about to consider borrowing a pair of mufflers just to eliminate one component from the short list of what is ailing.
I have a few weeks before we yank the new turd Dynamic converter, which we know IS disfunctional behind my motor.