1. I presume you bought the right impedance speakers for the head unit.

2. I presume the speakers a rigidly mounted.

3. I presume you have the speakers out of phase with each other (which is why they are sounding terrible!

To explain phasing...basically you have got the + and - muddled up so that rather than both (ok i know there are 4 , but for simplicity i will talk about 2) speaker kicking towards each other and creating a crisp shock wave, one peaker is kicking out, and one kicking back - this means the second speaker is absorbing the energy thus creating a muddy sound...

the easiest way to check is to pan from left to right on the front speakers, then from left to right on the back, then from front to back on either side..

the bass should get much louder in the middle, if it doesnt then the speakers are out of phase!

alternatively, take the covers off the speakers and get the speaker wires out of the back of the head unit.

for each speaker apply a 1.5 volt battery BREIFLY i repeat BREIFLY to the wires, you want to seethe speaker come OUT towards you - if it does then the wire on the + of the battery is the + of the speaker..

you want all the speakers to do this, then reconect with the + to the + feeds from the head unit

if they are in phase, then its down to mounting and impedance (unless the head just soundls like that - i presume you have tried altering the EQ settings on the head...)