Installing a stright through type muffler will reduce the rasp and will not affect flow and reduce power.

There's a few brands of mufflers that people have used as a resonator to control rasp:
Dynomax 4" round bullet race muffler
Magnaflow 4" round muffler
Eidlebrock RPM series muffler
Vibrant's resonator

I feel sorry for the people who don't drink. When they wake up in the morning, thats the best they're going to feel all day - former President Lyndon B. Johnson