Have you bypassed the valves yet to open up the turn-down tips? That made a considerable difference on the low rev rumble. Mine actually seems to be getting louder as the break in miles go by

I bypassed mine at 400 something miles and it made a noticeable difference. I now have almost 900 miles and its louder yet.
I would start with better headers and a non-cat. H or X pipe if you really want to get it loud. I will probably do that to mine once the warranty is gone. If you change the mufflers, you'll have to find somebody to do it custom with those center tips, unless somebody eventually makes a specific system for the '05