Avoid Blowing Speakers?

I don't think your gonna blow the speakers at 25 or 30, unless you hear distortion because whatever you have plugged into your aux port is cranked all the way up. if you hear distortion, that's "clipping". it essentially is sending a square wave to the speakers and that will damage a speaker even at lower power than what the speaker can handle. as long as it isn't distorting, the speakers should be able to handle it.

 

As stated, the main thing to avoid is distortion. If you hear any distortion, that indicates that either the speaker is being over-driven (i.e. getting too much power sent to it. This is unlikely to cause damage if only for short period), or the amplifier is being over-driven and is clipping (much more likely to cause speaker damage).

One reason why it's much easier to blow car speakers than home stereo speakers for example, is that car speakers are listened to with 70db or more of background noise (e.g. cruising on the interstate at 70mph), and that listeners often aren't focused or paying much attention to the speakers. Both of these things can mask distortion during playback.

For this reason it's best to learn your stereo's limit before you get on the road. Park your car in a quiet place (best is in a closed garage). Play a loud sounding, bass heavy track that doesn't have built in distortion (i.e. no heavy metal, dubstep, hard rock, etc.). Turn up your phone/ipod/whatever volume to maximum. Then starting from a low volume slowly turn up the volume on your stereo and listen closely until you just start to hear the first bits of distortion, then turn it down again until the distortion completely disappears. When the distortion completely disappears, this is the maximum volume setting you should listen to your music at. Note that if you change any EQ settings or use a different device, you'd need to repeat these steps. Usually this point is much louder than your average listener wants; On my non-JBL Prius this point was at least in the 40's for volume (I can't test now to compare however as I have an aftermarket CD deck in there now).

Also be aware that your speakers are close to 10 years old, and the speakers in the Prius are basically just made of paper and glue (even the JBLs). Therefore they will deteriorate over time. It's possible for the surrounds to give out purely from age, without receiving any abuse.