I honestly don't think it's the helmet at all. On my Ultra with SE slipons the sound resonated through my KBC Racer 1 to the point it hurt. I put the stock mufflers on for a trip from Wisconsin to Deals Gap and back and had no problems, the stockers are still on and I love it, I've grown to appreciate the more mellow sound. As a bonus I can listen to the stereo, and hear the voice prompts of my GPS with no problem (through the speakers not helmet speakers)
I know it goes against the grain of loud pipes are cool, but for the type of riding I do, the trips I take and the things I want to hear, stock pipes are the answer.
I am also a big fan of quality riding gear because I learned on the same trip that falling down hurts, so it's FF helmet everytime I ride and that is more important to me than pipe music. YMMV.