Upgrading my Soundcraft 8000 Channel Split

[Pedrosa]

Hello guys! I bought a sc8000 32ch for my studio, it's like new! Not a drop of rust. All the channels, master, aux, work perfectly. I would like to know how can i reduce the Hiss?, when I increase the Gain or the treble in the EQ, the hiss is very evident. I've already changed the OPamps TL071 to OPA2134pa but still the same. What upgrades can I do to make this infernal machine as clean as possible to record?