I have the 8" system with the Helmholtz resonator built into the system instead of the tail pipe. Keith recommended me do the tail pipe, but I guess I wanted to be different. It still has some drone at 1500 Rpms so Keith is sending me a tailpipe Helmholtz for free. I've just never seen anyone running 2 of them. I'm hoping it doesn't make the system to quite or cause more back pressure with 2 resonators on it. Keith has never tested this system this way so he isn't exactly sure how it will sound.