have no idea of what power the origial speakers put out, but they are definately not good - don't get me wrong, i'm not the kind of guy who drives around with the windows down and the bass up!!!
I have installed some Sony X-plosion (140W) speakers in the back my GTV, much better than the orig. but still not quite good enough - i really think that you will need a amp and some new speakers for the best possible sound. but an amp could work some magic...
good luck - and let me know..