I'm sorry that you are less than happy with your Milltek exhaust
The sooting up of 3 pipes rather than 4 was a batch issue but out of the 200+ systems we have sold it was a very small percentage. It is also not something that can be spotted quickly when the car is with us as the colour/soot will build over time but it could also explain your resonance issue. Milltek will replace any faulty system over its lifetime so you can have this swapped free of charge at any time.
Fortunately most customers are very happy with their Milltek and we regularly swap the system onto GTRs where others are to noisy and boomy to use day to day. However noise and what a car should sound like are very subjective so we now stock 8 different types of Milltek system (6xNormal and 2x90mm) so we can tailor it to the customer's tastes
We also offer 3 types of tail trim.
We will soon be uploading videos to our website which will hopefully show the different noise levels.