Oh man, I feel that... put a "performance" muffler on my old Chevy 350, and it just made this awful tinny rasp. Sounded like a giant bee trapped in a soda can, zero rumble at all.
So what kind of muffler did you actually get, @andrewf23? A lot of those cheap ones are just empty cans with no real baffles. That's why they sound so thin and raspy instead of deep.