I think your drone is because of the E Race mufflers.
The E race was engineered ( er... sorry ticat) designed to be an aggressive axle back muffler, for a then, fairly new platform.
I seldom disagree with Randy, however, I think in this case a resonated xpipe would sound less harsh, but not kill much of the E race drone do to the sheer volume of header back system.
There are many more options now, I think you can achieve better tonal balance and power throughout the rpm range by keeping your cats, non res x pipe and changing the back boxes.
You will want those cats anyhow, or your show car will stink. The worse the gas, the worse the smell. Esp. at cat heating sequence.
I absolutely love the test pipe header, secondary cat, non resonated x pipe and etronic box setup that Hasan and Imran , and you OP, helped lead me to. But not for M6, I don't think.
Amazingly little drone for amount of sound available.
Maybe in this application, Corsa is the ticket.
Your M6 is one of my favorites...