Car stereo guru needed.....

I don't know of many speakers that want an amp that put out more than the speaker can handle. I'm a DJ and you usually don't want an amp that is 1.5 times higher than your speaker just in case the output goes to max for any reason. I would say an amp that is close to the max of the speaker, even a little bit lower would be fine. This will ensure you can never overdrive the speakers. Square wave or clipping usually occurs when the amp is at its max output and starts to protect itself, not the speaker. If you find yourself having the volume up high all the time just to hear things, you may wish to go to a higher amp that would have more power and not work so hard but watch how high you go. Square waves are not good to send through a speaker because the driver goes from off to max and back quickly. This can tear things loose or fry the driver. Unless you are buying real high end speakers, you will not have a problem driving them at lower watts than they are rated. Highs do not take a lot of power like a sub or low frequency driver. High end speakers with a low frequency rating will want more power to drive the bass but most 6x9 and especially 5 1/4 will not go low enough to really worry about using a separate amp. Unless you want bass you can really feel and the speakers are setup for separate high and low inputs, what the aftermarket radio feeds them shoud be fine.