DSP is a small part of your issue... the drivers (ahem, "speakers") used are in a word... garbage (being a family forum and all). The M490 set up is actually the predecessor to the M680 / DSP set up. First things first... do you have a 6ch or 4 ch amp (this determines if your rear parcel shelf speakers are driven off the amp or head unit)? Remove the retaining bolt on it in the frunk and the big label facing firewall will say either 4x40 or 6x40. Regardless, you really ought to consider changing out the speakers (dash & door)...
Take a look at your amp and let me know. We can go from there. Or you can reference these two files on the Boxster Wiki page:
Upgrading the Boxster Sound System - by AndyM - very comprehensive
Template for the Upgraded Door Speaker Baffle - by some guy Burg Boxster
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