I have the same problem with my R32 GTR.
HKS Dump/fronts
3" cat
3" D-Luck cat-back
Big Canon
5" Tip
It is so loud and drones so much that it's too loud to even sit idling in my driveway or in the main street where I live. On motorways in 5th gear doing 100km/h at 3,000rpm it's incredibly painful to the ears (especially if doing a 400km trip). I got pulled over by the police last nite and its the first thing he mentioned (but let me off coz he loved GTR's).
I just had a closer look at the rear canon now and it appears to be a straight through pipe with some sort of mesh as the walls of the pipe. Their is a flange just before the canon so I guess I have 2 options from here:
1) Fit an Apexi (or variant) exhaust control valve for $250 fitted, which would almost guarantee a quieter exhaust but i'd still have to manually play with the valve everytime I drive due to loss of power when the valve is closed up.
2) Try a few different rear canons to try and find a quieter one until I get 1 that quietens it down enough whilst still possibly being above the legal db limit.
I have plenty of power (330awkw) so I don't mind losing 10-15kw if it means a quiet legal exhaust but obviously I don't want it to rob me of power and heaps of money either.