"I have connected my G-System and preamp or Head preamp with the 4 cable methods describes in the manual. However I cannot get the sound of my preamp."

This is because the insert loop is not active.

Press EDIT, then scroll to LOOP wit the Page encoder, press ENTER,
Set the Insert loop to 'ON'
If you want to have the loop permanently active, press MENU, then go to UTILITY menu and set parameter INSERT LOCK to 'ON'
Press EDIT to revert to play mode.