Quick followup on this...
I was able to find the HAES 4 channel amp used in my Boxster on Ebay. I figured this would at least confirm if the problem was due to the amp or the head unit. Today I tried swapping out my amp with the one I bought on Ebay; sound still very low in the one channel, telling me the amp is just fine

. So if I want to stick with the OEM sound system, next step is to find a replacement CDR-220 head unit. Also, if anyone out there is looking for the HAES 4 channel amp used in the 2001 Boxster (it's the non-MOST system), I'd like to sell the one I just bought since unfortunately it didn't solve my problem.
Thanks,
Ron
P.S. The Bentley manual is a fantastic resource!