When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Your best bet is to get some factory replacements if you are happy with the performance of the stock system. Due to Blose being well, well marketed and under performing, they choose to use non-standard ohm ratings for their speakers in an attempt to generate a "little" power from very small amps. Translation, the system is not very easy to plug and play upgrade, so you are correct.
You can upgrade the 3.5" "twiddler" speakers which are 4 ohms with several offerings from major manufacturers. This will improve your high frequency performance some, read threads here on the forums to see opinions.
You can upgrade the rear 5.25" or 6.5" coaxial speakers with standard drop in 4 ohm replacements as well, but you are looking at a pretty small (if any) sound improvement IMO.
The door speaker that is likely rattling is the 8" mid/woofer from the factory blose system. There is not a plug and play replacement driver available due to the 1.5(?) ohm rating. More bad news, the replacement speakers from a dealer are something like $300, find them in the for sale section from forum members.
G'luck and thanks for doing some research first
Fej
You could probably find a used Bose replacement for $20 if you create a thread in the FS section. Some people sell all 6 speakers for less than $50 over there. Post a WTB thread and you'll be set.