The old schematic you found belongs to the "real" Sun Coliseum 300 from the 70īs ... and it never had compression, nobody did way back then.
IF it does, then itīs something else, a "reissue" or something a new brand owner made to cash on the name.
IF made by Fender, they bought the Sunn carcass but continued only with the PA equipment, *might* have made some Bass amps, then they should have a suitable schematic, ask them i any case.
We are in complete darkness here, at least post some pictures, including any "Made by / Made in" labels, some gut pictures and some closeups showing any labels on the boards themselves, both silkscreen and copper side.
That said, "compression" and "16 pins" in the same phrase, make me think of LM13600, a very popular gain controlled OTA:http://www.b-kainka.de/Daten/OPV/LM13600.pdf
For starters check whether datasheet pinout matches what you have on PCB or not.
That said, a cracked IC makes me suspect supply overvoltage or some similar gross problem, check you have proper +/-15V rails or whatever that preamp uses.
FWIW that amp might be a generic Korean OEM amplifier with a Sunn label glued to it.