If the AMP1's FX Loop is set to parallel, you can connect a second audio source to the FX RETURN - in addition to the guitar connected to the INPUT. This works well in the case of another guitar preamp.

MP3 players, other instruments

You will achieve less good sound results if you connect audio sources that actually belong to a mixer with neutral further processing (mp3 players and other instruments, for example). The AMP1's FX RETURN goes straight to the power amp, and from there the signal usually goes into a guitar cabinet. 

Both power amp and cabinet do not sound neutral, but color the sound - sometimes quite significantly. What sounds good for an electric guitar or a guitar preamp does not necessarily sound good for a piano or an mp3.

In addition, the nanotube output stage of the AMP1 goes into compression/saturation from a MASTER position of approx. 6, which distorts the signal. And of course you don't want that with an mp3!

In the case of a stereo mp3, it should also be noted that the AMP1 has a mono output stage and the mp3 can therefore only output mono.

Conclusion: For practice purposes and/or if a good sound is not the top priority, you can use the FX RETURN to connect a second, external audio source such as an mp3 player or other preamplified instruments (acoustic guitar, piano, etc.) and this signal additionally to the guitar connected to the INPUT. However, you should be prepared for tonal compromises and not turn the MASTER up above 5.

Microphones

It becomes very difficult with a microphone because preamplification is missing here. The microphone will not only not sound good, it will also be very quiet. The only way to control the volume would be with the MASTER knob.

Conclusion: It is theoretically possible to connect a microphone to the FX RETURN (only a dynamic microphone, by the way, not a condenser microphone, because the latter usually needs phantom power), but the result will probably not be satisfactory.