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Question If you are using the G-Major in an effects loop with a +4 dB/ -20 dB switch, which value should it be set to

Answer
If you have set the Input range and output range of the G-Major to "PRO" in the I/O menu, then having the switch of your amp set to + 4dBu should be a perfect match. Of course you can use the input and output level knobs to tweak the levels if necessary. Make sure you leave some headroom at the input stage.