Bravo to you in shredding all hopes for the OP. The only difference between co-axials and components is the independent placement for the tweeters when it comes to the same make and models. Even if he changes to the "cheap" sony components, he will keep banging his head against the dash since those xplods will be as harsh, if not more, as these pioneers. Worse still, with the tweeters placed on the dash, his ears might actually bleed.
@OP
I suggest you go ahead and connect the front co-axials to the amp. Since you don't wanna add another amp, you can keep running the sub from channels 3,4 and let the rears run directly from the pre. Disconnect the sub, change the fader to the front while you are setting the gains from the amp for the front coaxials and turn up the volume to 75% from the pre. This is the maximum volume you will ever listen to music in your car so you need to set the gains accordingly. Preset all settings from the pre to 0 and set the gains for the front coaxials till the point where you feel comfortable listening to them and they are not distorting. If they sound a little harsh, don't worry. You can always play with the EQ later. Once satisfied, play with the EQ, particularly the mids and high frequencies. You should feel the harshness reduce and at the end of the calibration, you will actually feel comfortable with the music. Reconnect the sub ( if your pre has the sub on/off function, you can turn it off and on from there while setting gains for the front speakers ) back to the amp and set fader to zero. Remember, the rear speakers might still sound harsh since they are running from the pre so I suggest you set the fader to 60/40 or 75/25 front/rear. This will give you a more balance front stage.
Keep all settings like loudness, sla, bass boost to zero. Remember, your sub is the bass producing driver here and you can tune how much bass it produces directly from the amp.
Hope this helps. If you can't set the amp settings yourself, take the car to a audio shop and tell the guy what I wrote down here.