I run a Vance and Hines S4 stainless steel.
Sounds pretty darn good.
I compare it to messing with an EQ on a guitar amp.
The S4 added alot of bass grunt to the tone while boosting the mids and highs slightly.
Sounds like a beast at idle with a low end growl. Flick the throttle and you get a sharp bark like a rottweiller. At cruising rpm, it's louder, but not obnoxious at all. I had heard V&H were loud, but it's not so. At full on boogie, it's pretty noisy, a good combination of bassy grunt along with a mid-dominated wail into upper rpms. Only at redline does it sound a little tinny and hollow.
I haven't heard it from a distance though. Only from the pilots chair.
The S4 is a great pipe. Cost me $270 from Starcycle- but they took too long to ship it, IMO. Back ordered maybe?