My suggestion: If you think the stock (base or premium) stereo doesn't sound good, start by replacing the speakers with better quality ones before you worry about replacing the head unit.
For example, my '97 C1500 has the stock head unit in it. For 6 years I listened to the crappy stock speakers. I got bored one weekend so I went to multiple big box electronics stores and listened to their speakers they had on display.
I ended up ripping out just the 6.5" door speakers and replacing them with new Sony speakers (not the greatest but certainly within my hobby budget for the month). Suddenly I had crystal clear mids and highs but lost my bass (well, what little the stock paper speakers could put out). No problem, tossed a couple of 10" Kicker subs in boxes under the rear seat and wired them directly to the rear R/L speaker wires. I didn't add an amp or anything else to the stock head unit and the sound is great - clear, loud, and well rounded. And to think that I almost started with the head unit and hooked a new one up the old stock speakers. Total cost was less than $200 for the new speakers.
Start with the speakers and if that doesn't do it for you, then worry about the head unit.
By the way, this song does an awesome job of showing off the great range and clarity I get out of the system now.
Let it die