Another favorite glasspack type is Smithy mufflers, they can have nice tone for the six. Also they available from in lengths @ 22" to 30". Preventing the resonant drone is always the goal with the six. have fun http://www.speedwaymotors.com/Smithys-Glasspack-Muffler-30-Inch-Body-2-Inch-Inlet-Outlet,39835.html
Drone can definitely be annoying which is why I opted out of the cheater turn down at the muffler and brought the tail pipe all the way out the back. Not much if any drone.