everyone assumes its the speakers that are crap, infact its the headunits that are weak, I have a jvc 4x40w and it sounds great with the standard speakers, I changed the speakers for sony ones and it was worse, The black coned speakers seem to have slightly better sound than the beige coned ones tho.
The radio and the speakers were put in there by the manufacturers to hide the rattles of the car, and all have been built to the aboulte lowest budget possible.
The standard stereo is indeed very poor quality, and most models are less than 15 Watts per channel. So yes, a serious improvement can be had by replacing this.
However, the speakers, also built for a few pence, only have an input power rating of around 18 Watts, and often blow when the original stereo is connected, so what do you think is going to happen when you connect a modern 50 Watts head unit to an 18 Watts speaker - doesn`t take a genius to work that one out. incidentally, the Beige ones are probably nothing more than sun bleached black ones !!
Ideally, you need to replace the lot, head unit and speakers - but a little tip here, go for a quality set of speakers, as some of the budget stuff can spound just as bad on a good head unit as the OEM ones did on the original stereo - and another tip is to avoid anythng with a Sony badge on them !!
So far, the current Clarion range of speakers we have on our website seem to be very popular when fitting into the standard locations without breaking the bank - certainly worth a look !!