Thinking about replacing front door speakers

I have a 2013 Prius v wagon. Based on what I've been reading here, I think it has the JBL sound system- I have a 3 band eq with a front/rear fader/balance control. There are speakers in the dash and the doors.

I'm pretty sure one of the door speakers is damaged somehow. I get much more bass from one vs the other, and a little distortion from one speaker.

That being said, I actually think it sounds pretty good, especially for a stock stereo.

BUT

I'm not afraid to dig in and replace stuff. I've read of other users having blown speakers, and I'm led to believe that perhaps the speakers aren't the most durable, or perhaps they're not so well matched to the available power. I'm happy with the sound of stock in this case, but if I'm going to shell out some dough, I want a more durable speaker.

SO

Some questions come to mind:

1) Just how efficient are the stock speakers? How much volume will I lose by upgrading. I understand that all speakers have different sensitivity levels... but let's say I drop in something with 89db sensitivity. I have an amplifier on hand (I understand I'll need to get a signal combiner), but I prefer to leave things as they are if it will be loud enough.

2) does anyone know what the crossover frequencies are for the front door and front dash speakers?
Also, are they split at the underseat amp or at the headunit?

3) how about the impedance of the stock speakers? I don't want to strain the built in amp.

I probably have more questions, but I'm drawing a blank right now.