Chevy HHR Network - Better factory Pioneer speaker performance with cargo floor in "upper" position?

I recently took a 2007 HHR 2LT for a ride and found that the premium Pioneer speaker system was nice, but it does not have the "kick" of my factory PT Cruiser speaker system (with dealer installed subwoofer). While I was driving home, I realized that my rear tray in my PT is in the "upper" position, which in turn creates a semi-enclosed space for the subwoofer to resonate. The HHR that I drove had the cargo floor down, which in turn meant the subwoofer didn't really have a space to resonate. Have any of you factory installed premium Pioneer speakers owners noticed an improvement in bass when your rear cargo floor is in the upper position near the windows? I'm set on ordering a new HHR (maybe the SS) later this year, and I'm racking my brains out about the stereo. Thanks for your help and input! :)