Sound Help

[Essay]

Hi i have got 2 alpine 1242D 900watt peak 300watt rms running them off a alpine mrd301 mono amp 760x1, and i have no 6x9's or anything in the rear just an old set of my components but only using the mid bass due to having difficulty finding somewhere for the cross over and tweeter, this sounds great until i want to turn the music up loud and then it starts to sound a lil distorted like theres too much bass running through the speakers, what would be the best option to get a louder stereo, buy a good set of coaxials and amp then, or is there no need for an amp and just run them off the head unit? and also a suggestion of coaxials would be appreciated
sorry for the essay wasnt sure how to shorten it down