I’m no rocket surgeon…every studio session, I haveta try ‘n remember which way the pointy end of the guitar goes….
But I’ll betcha a bag of pucks that a pre-amp/signal booster is not necessary - the Yamaha’s line-level output should be completely adequate. It’s being attenuated somewhere - and most likely after the signal has left the KB.
For starters: check your cables; check your gain-staging; and be sure your M12 is set to take a line-level signal and is in the right “mode”.
You might haveta read your manual.
What @shredd said.
You should not need a booster to get a decent level of sound from a synth into the Model12.
Either you’re missing something/doing something wrong or the synth/mixer is faulty.
It’s easy to check if the synth is working properly by connecting it to some other device and it’s also easy to check if the mixer is working properly by connecting some other instrument to it.