The FX Loop Level Switch, which can be used to switch the sensitivity of the effects loop from -10dB for vintage FX pedals to +4dB for modern pedals and studio effects units, is located on the underside of the AMP1. It is designed as a push-push switch and sits in a recess to prevent accidental activation, which means that it is not directly visible at first glance. To operate it, you need a long, thin object such as the jack plug of a guitar cable.
Info: The AMP1 Silver Edition switch enclosure has a slightly smaller diameter - too small for a standard jack plug. Here you have to make do with a somewhat thinner object. On the AMP1 Mercury Edition and AMP1 Iridium Edition, we increased the diameter so you can operate the switch with a guitar cable jack plug.
By the way: if you don't hear any difference in volume when you press the switch, this is not a sign that the switch is defective, but that it works. You can find out why this is and what exactly the switch does here: The FX Loop level switch does not work.