a bit of advice mate, when it comes to audio (infact, anything) go for quality! its the garbage in, garbage out principal. heres an example...
you have a super duper 500bhp engine, but fit the car with plastic wheels - you're not exactly going to reap the benefits of the engine are you
you have a 30bhp engine, but have the latest in super sticky tyres fitted, see my point?
even if you replace the speakers, and fit a new amp, your poor sound is still probably coming from the head unit, all you would be doing is amplifying the dirty signal, and forcing it through the speakers. regardless of amp or speaker quality, if the signal coming in is crap, then the sound coming out will be amplified crap.
my advice, brand names dont exactly mean quality (so dont hunt for them just because of that). get a very good quality amp, something a bit too overpowered, because odds are, once you start, you're going to want to go louder...plus it will help retain clarity at higher volumes. the amp is where youre going to get the biggest issues, if its strugling, its going to clip, if it clips your going to burn out your speakers, so choose well.........then after buying an amp, if you're still not impressed with the sound, swap test speakers with another set, if its the same rubbish sound, then replace the HU.