Of course, Commodore being Commodore (notorious for not sticking to spec and instead frequently just using whatever parts they had in stock or could get a good deal on), there are C64C's with "breadbin" motherboards and breadbins with C64C motherboards (C64G, etc.), and while most C64C's have the 8580 SID and most breadbins have the 6581, this is not true 100% of the time. You really have to open any given C64 to see what you're actually dealing with.
Well, the thing about the SID in particular, is that it relied upon imperfections in the MOS silicon fabrication process to produce certain analogue effects (a fairly common approachh in electronic instruments in the 80s). Problem is, that as processes improved it gets harder to produce noise in this way. The later generations of SID produced a “cleaner” sound, but not faithful to the original to the point that certain effects exploting these imperfections (eg some funky sound sample techniques) were broken on the newer version. I’ve read somewhere that it isn't possible to produce the SID at all with modern technology because processes are just so “clean” now.
