Here's how to shut down the Preamp section Globally in the GT-10.   

1. Press the SYSTEM button.

2. Turn the dial to choose the "PLAY OPTION" icon, and then press ENTER.

3. For the PREAMP MODE, turn the dial to select SYSTEM 1, 2 , or 3. (It doesn't matter which number  you choose).

4. Press the PREAMP button either once or twice until it is not lit- (This turns off the preamp). 

5. Press EXIT. The new setting is saved automatically in the System number you chose in Step 3. 

The preamp will now be set to "off" for all patches.