When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Re-doing the stereo on my C5 was one of the best mods I've done. I replaced everything, but I think you can change the 3 1/2 speakers up front and the rears for a bit better sound. But those guys know best.
I recently did mine. I changed the stock head unit if you have the bose system count on spending 100.00 for the harness alone then a few bucks for the antenna adaptor and the faceplate.
If you don't have the bose your in luck the wire harness is much cheaper.
The bose speakers clip really easily with an aftermarket head unit as well which sucks. To make mine sound somewhat respectable I added 2 10" subs in a box in the back which I will change out when I have time to make the box fit in my spare tire slot.
You can have an OK sounding system but nothing to great without external amps and changing the speakers out. I would at the least if you are going to do anything change the head unit and the front speakers and add your own amp. Then you have complete control. Try to find a component set with a 6+ inch midbass driver and tweeter. Honestly FOCAL is some of the best sounding stuff out there.
Like others have mentioned check out the audio/electronics forum under the general corvette tab. Then read the C5 audio faq first and foremost as many basic questions are covered, then check out the "what do you run" thread for ideas and budgets. Lots of guys in that section with varying degrees of stereo in their rides that will help.