Well, if you don't have a sub then I would say stay on head unit power (if you stick with 4x10's).....unless you have an amp that has at least a 24 db/octave x-over on it....Those spkrs just distort badly when you push power to them in the lower octaves.
IF you have an amp with a 12 db x-over then you have to move the x-over point up....then your music just sounds crappy....
I would say go the 6x9 route, they handle the low stuff MUCH better, and there is a wider variety of brands to choose from.
Boss is the best brand I've came across for 4x10's that will handle low notes w/o crackling.
just my .02 from past experience with those dreaded spkrs.......