98 premium sound speakers

I've got several speakers rattling in a 98 GTP with premium sound; i.e., two 6x9s and two 4" speakers on the back deck and 5 1/4" speakers in each front door (as well as the little tweeters in the corner of the front windows). I've gathered that there's an amp behind the glove box and the schematic shows that all speakers are wired to that amp. The signal to the 6x9s are apparently low-pass filtered somewhere.

It seems that the factory stereo will provide satisfactory volume and bass thump if I can get the speakers to quit rattling. So I want to replace the speakers but keep the factory head unit and amp.

Are all six stock speakers 2 ohms? (I can sacrifice the little tweeters in the window corners...they don't seem to add much in my hearing.) Does the separate provide boost for all speakers or only the 6x9 woofers?

Does anyone know what the cutoff frequency is for the 6x9s? It seems that anything more than a two-way speaker may be wasted.

Does anyone know the cutoff for the rear midrange/tweeter?

Are the front door speakers full spectrum?

Any recommendations on speakers or specific favorable attributes for speakers in this configuration?

Thanks for any help,