According to true 6581 afficionados, the 6581R4AR's are the best sounding 6581's out there. They're "4th" generation (hence the R4), and used in most of the c64c (the beige new-case styled c64's, as opposed to the oldschool brown breadboxes). Some C64 musicians (most notably Jeff/VRZ/CML) adore the sound of the 6581, and think the filters and waves this particular SID produces are way better than the 8580's. The 8580's are preferred by the more techno oriented musicians (like PVCF/Reflex...), because of the sick filters and some extra waveforms (hey, that's why most people here seem to prefer the 8580/6582, because of its ability to get dirty like a 303........ which the 6581 can't do because of some well, hmm, it misses some of the extra waveforms in the 8580, and the filters aren't that good). Hope to have helped you a bit :D I prefer the 6581 as well, just because of its cleaner sound, and its ability to do digis using $d418 register on the c64 ;D (Hmm, you can't tell I'm an avid c64 user can you? 8) )