Mixing consoles don't have space to house top-notch (and expensive) discrete preamps. The Soundcraft preamps may be, well decent – that is, sufficient for any typical source: condenser mics and active DIs already pump the line with considerable levels, whereas dynamic mics and passive DIs are mostly used with instruments that are so loud that you don't need a lot of gain in the first place.
That doesn't however mean that these preamps give good results with any signal you could want to amplify. For one thing, they have too low impedance for guitar pickups etc., but they also produce a lot of noise when you crank the gain. If that's necessary for your application, I'm not surprised that you have problems there.
The way to go is to run these piano pickups with a suitable low-noise preamp. A good active DI might do the trick. Depending on the impedance, any buffered guitar-effects pedal (in bypass but not “true bypass” mode) might help. Else studio mic preamps will work – those are designed to amplify even quiet passive ribbon microphones without too much noise, and those are probably quite similar in signal to those piano pickups.