You can get decent sound with an aftermarket head unit, stock Mach amps and speakers, but just add a sub amp and sub. That's how my Cobra was set up when I got it. It had the correct wiring harness for the headunit that sent the speaker level inputs to the Mach amps, but it had the RCA outputs for the subs ran to an aftermarket amp powering a sub. It actually didn't sound too bad. Compared to most factory stereos, not bad at all.
Now, since I'm a huge audio fan, my Cobra was recently de-Mached. I removed all the factory speakers, and the huge contraption from under the rear deck with the amps, speakers, etc. hanging from it. Then ran all new speaker wiring and the front/rear RCAs to the new amp. I made a set of 6x9s fit in the rear deck cause I had an extra set, then made a plate and put 6 1/2 woofers in the doors, with the seperate tweeters up where the Machs were by the mirrors. Even mounted the crossover for the components in the little opening in the door since they look nice. Not too hard of a do it yourself project, and relatively cheap to do...just time consuming it you want to do it right. O0