The FX Loop level switch does not work
The FX Loop Level Switch, which switches 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 in a recess to prevent accidental activation. To switch this push-push switch, you need a long, thin object such as the jack plug of a guitar cable.
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. This is because the switch raises or lowers the level of the FX Loop on both the FX SEND and the FX RETURN, which means that the overall level of the FX Loop remains the same. This means that the switch is not intended to make the FX Loop louder or quieter.
Instead, it is used to prevent effects devices or pedals that are primarily designed for use in front of the amp (e.g. the NUX Time Core, BOSS DD2 or vintage preamp effects in general) from starting to sound muddy or distorted because they cannot handle the FX Loop's Input level (see also muddy-sounding effects device in the serial FX loop). If the FX Loop Level switch is set to -10dB in such a case, the problem is usually resolved.