This has been a long time pending, fortunately I was able to finish it in time for AITP! Already written a detailed write up as a follow up i the ICE section (low rest but PIC HEAVY):

A4 B7 Cab Audio Upgrade...

The problem with the A4 cab (compared to the E46 for example) is that the rear backrest is a *&%$ to remove, so fitting a box directly to the bulkhead in the boot is a difficult option. I just wanted to add a little bit of bass at the low end which is missing from the factory system. Still breaking in but I think it's done the trick!

Comments and questions welcome