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Define loud. Are you looking for a throaty race sound? How about a low rumble? Is interior resonance an issue? I love the sound my Bassani gives my truck, but I do have some slight resonance (the muffler is basically right behind the passengers seat on the broncos). Flowmasters will probably be pushed by everyone. I dont have anything against them. They are loud at the tail pipe, and loud in the cab. There are other manufacturers, but I do not have any personal experience with them.
If you want loud the best and chepest way to get it is to run straight pipes. But being loud doesn't mean it will sound good. But straight pipes can sound good.