Exchange 2 Ohm speakers to 4 Ohm ?

[spongebob]

I have so far done the three first recommended steps on my way to a decent sound in my Boxster
Installed is the original 4 channel Haes amp plus:

-New front speakers
-Installed PNP rear speaker kit
-New 6" door speakers

But sound is still more or less crap and as soon as I set Bass level above +2 there is severe distortion. Think it is the door speakers that can not handle the low end range. I have tried a few different speakers and today I have those installed:

SXT6 « InPhase Audio
They are 4 Ohm instead of the replaced 2 Ohm original ones. Is this causing the problem and should I look for a set with lower impedance?