mark b....(my opinion)
i have quite a loud sound system in my VX (running about 1500watts RMS to the sub)
i have stock speakers and my sub will distort before my speakers will...in my opinion if you put in a GOOD 4-channel amp the stock clarion speakers sound great and wont distort...
honestly i can crank mine up to be heard from a couple blocks away and the speakers wont distort for anything...they are actually pretty solid =)
if your going for a different sound than stock then change the speakers, but its a consideration for you because it will cause your sound quality and volume to go up...