s-flow + bullet still too loud? Mayb ur muffler or bullet fibre oredi bakar? From my experience, don't use 2 bullets as it will effect your drivibility. Although it does damped the NV but there seems like a slight lag from stop and go and your pick-up suffers.

Mayb u can replace ur bullet with a heavily dampened muffler or replace ur muffler with a mugen twinloop (or a replica)   

U can only shift ur powerband with basic mods, to extend u need to spend major $$$.

I heard from the exhaustman that the placement of the centre bullet does effect the powerband: Closer to xtractor - better pick-up, further from xtractor - better topspeed, is this true? But then this was the same exhaust man who told me 2 bullets gives more power