Alessandro,
I agree with John. For us to be able to help, we need to know what it is doing and what the problems are.
Do any of the lights or display come on?
Do the amplifier LEDs come on? When you adjust the gain control, do the LEDs change?
Give us a good description of what is working or not working and we will try to give you some advice. If nothing is working, that is a clue!
Do you have a voltmeter? Or could borrow one?
If you could take pictures of the top and bottom of both circuit boards, maybe one of us will spot the problem. Post them here.
Another suggestion would be to go back through the entire parts list and verify each part. Check the direction of the parts that matter like ICs, big capacitors, diodes, the voltage regulator, etc.
Greg H.