advice on audio set-up

Juleous

If you were going to just add a Sub with a cheap interface then go for it, if your going for the fullrange upgrade with the components etc. then don't do it. The sound you get from a cheaper OEM interface is not great for the mid to high stuff, to get good clean sound your looking at a Clean Sweep or Bitone and that is as much as your Nav unit or close anyway.

I found the sub upgrade to the stock sound is a good investment, then trying to add a sound stage to that not so good.