If you're looking at upgrading your whole system, my personal preference would be a milltek system, although as already said, they come at a price. All aftermarket systems are louder than standard, especially with non resonated, big bore pipes, which are more about noise than efficient performance. Milltek (and Jetex) systems have a resonated option, which means that there is little extra space in the pipe when the gases are slowing though for the system to resonate, echo and boom.
If you want a cheaper set up, go for a full VW system, which will be quiet, and will give standard performance. However, it sounds like in your situation you just want to change the backbox, which I would do with a replacement Janspeed one. Janspeed are a reputable firm, and again, I've heard there exhausts aren't incredibly loud when you don't want them to be either.
Hope that helps!
