If you really want to have loud, and I mean loud, you could go as far as MSDS headers, or gutted pre-cats, off road Y pipe, or true duals, not install any muffer and just run resonated tips.
Or if you wanted something a little different and don't mind the look of them, I'd suggest the above mentioned set up but instead of the resonated tips install a set of supertrapps as the rear mufflers. This way you can add or remove difusers to "tune" the sound to your liking, and if you are really feeling wild you could remove the difuser assembly all together and essentially run with no muffler at all. I think, at the old site, there was a video floating around of someone with a true dual set up running supertrapps, and it was pretty loud and sounded nice.
another way to go would be to run a similar set up but install a Magnflow 4" round muffler in the position of the stock resonator and not use any rear mufflers.
