I think you will find that the 'woofer' element of your rear speakers are two ohms, but the tweeter element (its two speakers in one), are 4 ohms. The fronts are 4 ohms too IIRC
If you are thinking of amping the rears, you might be better re[;acing the standard 6x9s...
I was thinking of installing the following amp for the meanwhile:
http://www.beclever-shop.de/s03.php?sid=3f...pp=aa&bnr=32004
and powering the front speakers from the apmlified headunit outputs.