I disagree with TomC and would encourage you to replace speakers first if you truly think you need to upgrade the sound in your X. BTW what exactly do you mean by improving your sound level? Volume? Clarity? Bass/Treble response? If you want more volume, get speakers with higher sensitivity. Crutchfield does have a good explanation for this rating, its something like for every 3dB increase in sensitivity in a speaker you need only 1/2 the wattage to gain the same amount of volume, or something.

There are probably stereo shops in your area that would let you do a comparison between your existing speakers and the ones they sell, to give you a better comaprison. If you want better clarity at higher volume, get an amp. Personally, one of the reasons we bought our X is because we thought that the stock system was great, with no immediate need for upgrade. Each to his own.

------------------
2001 SE 4x4 5-speed
Super Black