I remember Joe Donnelly replying to a thread saying that he thought the Walker Muffler (he gave a part number) was the best sounding muffler he had heard. Deep throaty sound, I believe.
I don't know if you can do a search and find that post or just get ahold of him. You might want to ask where in the system he placed that muffler, also, and if that makes a difference.
I put a 14" Magnaflow just in back of the header pipe ( almost under the shotgun seat). These new trucks sound a lot different than the older ones. Sound more like a sports car than a diesel truck.
Also, see if Joe Donnelly knows how to get rid of the drone. I've got it at about 2100-2200 rpm's. It's not too bad without a trailer on the back but, if you are pulling up an incline with a load on, it's pretty loud. I've heard that if you install an additional 11" Magnaflow in the system it will take away the drone. I like it a little loud, just wish it had more of a throaty sound.
Hope this helps.
Joe F. (Buffalo)