Sorry, as you can probably tell from my first post, it frustrates me the amount of mis-information flung about on here as fact when it comes to audio equipment.
OH the Irony!
your giving out duff information as well
underpowering a speaker will not kill it - trying to give it more power than your amp can produce will but not underpowering it
when you have your volume turned down low you are underpowering you speakers, do they die? NOPE!
you can kill a speaker in 2 ways, mechanically or electrically
Whacking a load of power into it will cause it to over excurt and you will either rip the surround/spiders or the coil will pop out of the gap or slam into the back plate,
also too much power can cause the coil to blow.
you can also kill a speaker with clip - that is when the amplifier stops producing a nice smooth ac signal and starts giving the speaker a 'clipped' dc one. this will heat the coil up pretty freaking quick and it will either melt and go open circuit (which equals useless speaker) or it will go short circuit (which might kill your amp if its not got protection)
You WILL NOT kill a speaker by giving it too little power
in answer to the OP
Your stock speakers wont be able to take much power most headunits, (either factory fitted or aftermarket) will probably only give out 5-10wrms.
any low power 2 channel amplifier hooked upto the stock speakers will sound better than the built in amplifier in your HU. The trick will be finding one that does 2 x 10 or 2 x 25wrms
you could get a bigger amp but you would need to be careful with the volume on the headunit.