is there any particular reason why you want to reduce the noise?my friend had one on his starlet and i thought it was ideal,quiet at tickover and like you say below 3k but above that on full chat it was nice and sounded good.imho thats perfect,thats the ideal setup for road driving and especially in and about villages.not to anti-social:drive:
the only way i can think of is to incorporate more silencers and or reduce the diameter of the back box outlet,possibly fabricate a bung for fitting in the outlet.