I had the same experience :ah:
I am assuming that you followed the instructions and feed the rear speakers directly from the back of the CDR 220?
My next step was to upgrade to a 6 channel amp from a 996 - with not much improvement
The speakers finally came to live after I threw out the CDR 220 and upgraded to a new after market head unit where the built in amp had enough umpff
Now the front and rear speakers are driven by the new head unit and only the door speakers are driven by the amp in the frunk (with a pre amp in the middle)