Call around to some of the vendors, TTI is draconian about discounting their product, they will drop a vendor immediately if they find them discounting, but as one vendor told me, "they can't stop me from giving you free shipping". Ask for it, if they won't do it, call someone else, I recommend Mancini.
One of the other posters mentioned about another muffler only being a few decibles louder... Db is not linear, 3 or 4 Db is a very big difference! Also, just tell them which Hooker headers you have, the only difference in their systems is the short transition piece that goes from the collector to just past the crossmember cutout.