To keep factory HU or not?

I'm kicking around the idea of upgrading my 99 Jeep Chereokee's system. It's equipped with the factory CD w/ Cassette HU tht also has Bass, Mid, and treble controls, along with 12 presets.

I'd like to replace the factory speakers and use my old Kenwood 4chl. amp to power them, and also add 2-12" subs and an amp for them.  How much of a gain would just swapping the  Headunit out  be compared  to using line level converters?   I know it would probably be alot easer install wise, to use an aftermarket headunit,but I'd hate to let a half decent factory unit go to waste. 

or is there anyone who actually "beefs up" factory headnits and possibly add pre amp outputs to them?

I'd never downgrade to an aftermarket radio in one of my cars.  Sure, an aftermarket radio makes the installer's job a little simpler------- but you're stuck with a small radio, with tiny, confusing buttons----every one I've ever seen has obviously been designed by idiots who have never operated them while driving.

If you find an installer who has access to good line-out converters, and has experience interfacing with factory radios, I think you'll be far better off going that route.