What he said. If they go bad replace them. Folks used to think the Signetics were the best sounding 5534. There was a time in the eighties after the Gs came out that people were trying to hot rod their Es buy putting in op amps with faster slew rates like the Burr Brown OPA627A. Also increasing the 100uf coupling caps with 220uf. I know of a console here in Nashville where the tech only did those mods to the left half of the console. Nobody seemed to notice a difference, so he never finished the right side.