/6 single or dual exhaust?

Here's what I did. I bought a flowmaster for a Silverado which had a single intake and dual outlets. I put a set of Megaphone tips on it as well. My muffler guy said it will sound like crap but installed it anyway. To his surprise it sounds like a bigblock. Everywhere I went people allways asked if it had a 440 in it. It seemed to lose a little bottom end but gained some up top. I will do it again except no megaphoneys and tuck some turndowns behind the bumper. I used stock manifold.