New Speakers

[oily boyd]

Clean power from a high quality amp won't hurt those speakers, but distortion from a [censored] amp (even
Buy your 75W amp and simply refrain from turning it up so loud that the sound is distorted. Don't switch in the +8dB bass boost, either.

If you want to keep others from hurting the speakers, turn down the gain on the amp so that max volume on your head unit gives you good sound with no audible distortion.

 

[Virtuoso]

+1 on oily boyd's comments.

 

[kkreit01]

Don't worry about speaker rating. Just keep your gains low. Use the front 2 channels to run the front speakers. Bridge the rear 2 to run a sub. Keep the rear speakers on deck power. You'll probably also need an LOC -- which is real fun to hook up in the Focus. The easiest place to tap unto the speaker wires for high-level LOC input is bottom of B pillar (where rear wires go into door). It's a huge pain to get behind the OEM stereo (you have to remove half the dash + console). The Focus was the least-friendly car to upgrade for me (the door panels were a huge pain to remove as well). OE manufacturers keep making it more difficult to change anything out.