Technics sx-pr604 piano makes a constant loud hissing noise when on, everything else works fine. Is this hard to fix?

This could be many things. It is most likely noise from a defective chip in the unit. If sounds are normal except for the hiss, then test if the volume control reduces it. IF not, then problem is in the power amp section. If it does vary the loudness, then problem is likely between the D-A convertor and the volume control.

I would first try to isolate this by cooling selected parts with circuit cooler mist to see if you can find one that reduces the hiss when you cool it... Spray, then wait ten seconds before moving to the next on.

USUALLY semiconductors cause this, but rarely a bad resistor or cap can as well.

I was struggling with the same problem with my friend piano. You need to check the module which is plugged near power transistors. I found two bad transistors g109 and q119 (2SA1015). Changing those transistors and remaking soldering joints solved this problem.