who has changed out speakers on a JBL equipped Prius?

might want to check out the crutchfield website and use their fitement guide

 

4 ohm speakers won't damage anything. but everything else being equal, you will lose some volume. Look for speakers with a high sensitivity rating and that will minimize volume loss.
Speakers Glossary

 

As long as you can get the volume you want out of the 4 ohm speakers with your old amp, it will be OK. If you aren't using a lot of volume now you will probably be OK but you won't know until you try it, in any case it won't hurt your amp.

 

I swapped mine out about a month ago with some Infinity Kappa speakers. I did lose a little volume, but they sound much better. For reference, the tunes I played at about "15" volume with the JBL speakers now play at about 20-25 with the Infinitys. I've never had any reason to max it out.

I did disconnect the front door tweets (too much hassle to replace, so they just sit there), and replaced the rear door ones. I'm still looking for a good replacement for the center dash speaker.

The biggest problem I found was the shallow depth of the face of the new speakers. There was a gap between the new speakers and door panel, so the inside of the door filled with sound. I made a tunnel out of foam tape that now sends the sound straight out. Sounds great.

Nav, Bluetooth, etc. all still work fine.

 

The stock tweeters and center channel still work. The front tweets will stay unplugged, and I'll get the center going again when I find a good replacement. With the center going again, I think volume will match stock again.

I was just lazy and went with the most common set I found after some quick research:

http://members.cox.net/scott.sumners/Prius Speaker Replacement by ~sparky~.pdf

and they were a good deal through CarToys at the time.

The 62.9i set I put in the front has nice dome tweets in the center. I never like the sound of the ones up in the corner of the door anyway, so I just unplugged them. I can plug them back in, and that adds some volume, but they don't sound nearly as good as the others.

The 60.9cs set worked well in the back. The 6.5's give good bass back there. I just enlarged the hole in the upper door for the tweeters and used the Infinity mounting bracket. I think those tweeters are my favorite part of the setup.