My suggestion...
Before you go buy a new head unit, buy a nice pair of 6 meter RCAs and drop the $35 for the David Navone LOC. You'll need the RCAs anyway when/if you buy a new head unit. Connect the LOC to the stock system and run the RCAs. If you are not satisfied with the sound from the LOC, then sell it to someone here (or return it) and buy the head unit.
Factory stereos have improved dramatically in the past few years. Especially for imports (no comment on the Delco GM stereos). They put out a clean signal at moderate levels but break up, like any unit, at higher levels. A good LOC will allow you to use the clean low-output signal to power your amps and you will never have to worry about turning up the head unit to the point where it distorts. A LOC will also give you a stronger signal for your amps than a standard aftermarket head unit (9v opposed to 2-5v). If you want crossovers, equalizers, digital surround sound and 24bit DACs in your unit, then obviously this isn't the best route...but for most installs, a factory cd player will sound fine.