I like the XR1, but it's definitely pretty loud with the 6.0. It doesn't really drone too badly, but there is some resonance around 1500-1700 RPM, partly due to my exhaust leaks though.
This is the one I'm running:
http://www.summitracing.com/parts/BOR-40944/
This is the one I'm running:
http://www.summitracing.com/parts/BOR-40944/