This is because the 'Headphones out' parameter on page 9 of the Global mode is set to 1&2 ( refer page 39 for details ). This is the default setting so that outputs 5&6 can be used as Headphones that monitor the master 1&2 outputs. Simply reassign the parameter to 5&6 and these out puts will behave as outputs 5&6 and no longer monitor as headphones would.