Centre muffler or resonator does help a lot for the sound, as you found gets rid of the shitty raspy noise.
My R31 is a simple catback and it sounds mean, I had a proper oval type resonator fitted and a 'sports' rear muffler.
My R32 GT-R has some shitty barely there hotdog resonator and a half decent rear muffler, I rekon my R31 sounds better.