I don't have stock subs (uk spec, non DSP) but I am guessing that any improvement would be minimal due to the size of the subs and the power of the amp. It is very straight forward to wire up an aftermarket amp & small sub and you can even fit an 8" sub in the side area of the boot where the factory sub is mounted. Personally I put a 10" sub with built in amp in the main boot area which I got off ebay for very very cheap (second hand) and the bass is as immense or subtle as I want it now. The price was, as I say, so low for such a huge improvement but saying that I didn't have any subs in there to begin with...