I have run both, and have a fully resonated miltek on my mk4, and on the mk4 is is boomy as hell.
But on s3 the none resonated is excellent, no boomyness at all, and the resonated is too quiet by a long way.
I was at APS one day, a guy was having a miltek on his TT. He heard this noise as a car came of the ramp and got all excited, "WOW, that sounds excellent, I cant wait to drive it" he said........oh the dissappointment on his face when they told him it was mine!! They then moved his out and he was gutted!! !
Trust me, the rear box oin the s3 is so enormous. the centre box is not needed. Also, it runs quite close to the prop and in heavy rain when its hot, the system distorts as you hit puddles and it catches on the propshft, (other reason I junked it!!)