The gearbox in my B5 S4 has the much documented notchyness caused by the tight tolerances of these units. The cost of replacement for the refined versions seems to be prohibitive (in the order of £800), then there's the cost of fitting it all.

There's a lot of great stuff about the box problems and developments here:
http://www.jhmotorsports.com

However I wondered if it was possible and well easier to just purchase a later Audi 6speed quarto box and swap it over. Has anyone done this? does anyone know if the boxes have remained essentially the same.