So I was digging around on Wikipedia about the C64 and found this:

Quote Originally Posted by Wikipedia
There are two versions of the SID chip, the first version was the MOS Technology 6581, which is to be found in all of the original "breadbox" C64s, and early versions of the C64C and the Commodore 128. It was later replaced with the MOS Technology 8580 in 1987. The sound quailty was a little more crisp on the 6581 and many Commodore 64 fans still prefer its sound. The main difference between the 6581 and the 8580 was the voltage supply: the 6581 uses a 12 volt supply, while the 8580 required only 9 volts. A voltage modification can be made to use a 6581 in a C64C board (which uses 9V).
Now I'm convinced that I must have a later version of the C64C. I bought one (used) back in 1988, but lost it in a lightning storm in 1993, when I promptly replaced it--I noticed that the keyboard was different (how the letters and symbols were printed on the keys, I mean). I wonder if I had one with the original chip first. I'm basing this on the fact that when I play SID tunes on SIDPlay on my Mac, the sound is really crisp and clear, and listening to the same music on my C64C, it sounds a bit blurry.

Is the keyboard the key to distinguishing between these early C64C's with the "good" SID chip and the later ones with the "bad" one in it? I'd like to know which version I have--is there a website that shows the difference? I know I could get the original "breadbox" version to eliminate all doubt, but I really prefer the C64C keyboard (and my dust cover is specifically for that model anyway).