Was still curious to know if you notice any droning at certain rpm's?
http://dictionary.reference.com/browse/droning
That large open cavity inside the DPF shell... I'd expect some type of resonance to occur somewhere in the rpm band. A similar sized cavity in the exhaust of my Cummins inline 6 cylinder diesel caused a really unpleasant drone at ~1900-2100 rpm's. Ended up replacing the high flow muffler for something else to make it go away.