Thank you guys!
Here are my experiences so far:
FX send -> external looper-> FX return
works, the engaged signalchains have to be adjusted (FX loop on, tweaking a little bit send/return dB).
Guitar > FH1500 guitar input | Mainout > JamMan | JamMan out > Monitor In
does not work, the signal which my FH1500 delivers to Mainout is distorted. First time I thought it's my looper that distorts the signal, but it's the main out. Sound coming from the speakers is good, but sound from main out xlr ---- ouch.
To proove that it is the main out, I tested FH1500 main out -> Aux of a Bose Soundlink Mini and FH1500 main out ->Aux of a Yamaha THR10. Signal coming from guitar signal path is alway distorted. Very strange: only the Guitar/bass-signal is distorted. Music playing via firehawk app is ok. I can reduce distortion only by setting guitar level in the output level menu (or volume knob pressed 2x) to 20% or so, which is not what I like to do.
Nick, do you have an advice for me? Something wrong with my FH1500?