I repacked my burned out D&D's with "Silent Packing" from Vance & Hines. You can wrap 1 full sheet on each baffle. Total cost $16 bucks. It made them much deeper sounding & doesn't get real noisy until I hit about 4200 RPM. At highway speed & a half helmet the pipes are just a rumble.
BTW: My freak sounds like a vacuum cleaner under the seat when it starts pulling hard... I love it!
