The REC OUT of the AMP1 has two different application modes, depending on whether a loudspeaker is connected one of the amp's SPEAKER OUTs or not: headphone mode and speaker mode.
When there's no cabinet connected to the amp's SPEAKER OUTs, the REC OUT is in headphone mode. As soon as a jack plug is connected to one of the SPEAKER OUTs, the AMP1 switches to speaker mode and changes the volume.