Exchange 2 Ohm speakers to 4 Ohm ?
 

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?

Its ok to use 4 ohms speakers, you need higher wattage door speakers the one you have right cant handle the output of your amp.

Sure? manual says 240w max, The Haes is on 4x40 only

Try using a low pass filter capacitor inline with the door speakers that might solve the distorsion if not just keep the bass level to a minimum.