I think there's a subtle difference between the JRC and TI 4558's (FWIW I prefer the TI) but neither will change the general sound characteristics of a Tube Screamer significantly. Also, BOTH are "correct" for the TS-808 as either chip was used in vintage TScreamers. Other ICs can change the pedal for better or worse depending on your opinion...and are farther away from the "classic" TS-808 sound. IMO, the best bet is to socket the IC and try out different chips and let your own ears decide. Perhaps the biggest "Myth" is the TS-808 itself. I've never thought it was as great of a pedal as people seem to think it is and until recently I wouldn't even think of using one over a different OD or just plugging straight in and cranking the amp. I traded an original TS-808 for a script logo MXR Dist + back in the day as I thought the TS-808 sounded like **** through a Marshall. Now years later I sometimes run a TS-808 through my Deluxe Reverb or Silverfaced Champ and think it doesn't sound half bad.