After listening to the stock CSII system belt out crackly garbage for the last 10 years (time flies I guess) I finally buckled down and decided to change the stock speakers. Actually, I figured I'd replace them while I replaced my outer dew sweeps with the repros I ordered like 3 years ago...
anyhow, the stock 5.25" speakers I had in the door were p/n 16054454: A 4 Ohm speaker found in other late 80s-early 90s Buicks. With the 10 Ohm 3.5" dash speakers wired in parallel (stock configuration), the deck was subjected to only a 2.85 Ohm load. Bad. I ended up keeping the stock deck, stock 5.25s and just replacing the in-dash 3.5" speakers with more modern 4 Ohm ones. I rewired the fronts in series so the deck would only have to deal with an 8 Ohm load. Overall, it sounds much better now. Not amazing, but definitely acceptable.
The door panels had never been off. I searched for the p/n on turbobuick.com and it seems it happened to at least 1 other member there. I wonder if they just ran out of 10 Ohms on the assembly line and put in what was on hand.