For the katana series, you actually have options, however in all cases I would suggest that you have your output levels set to the default of the FM3 of -10dBu, and use OUT2, preserving OUT1 for use with studio monitors/PA/headphones.
Your options include..
1. The aux input (not present iirc on the 50W versions).
2. The FX return (as you have decided). From my understanding, this amp series does adjust the tonality of the power amp according to the amp chosen on the front panel so there may be tonal artifacts.
3. A solid alternative, is to set the Katana to PANEL mode, plug into the guitar input, and select the ACOUSTIC amp model. Set tone controls to 12 o'clock and fx controls fully anti-clockwise. Using this method has a benefit of adding the tone controls of the katana into your signal flow for quick adjustments
In all cases,
1. I would recommend running some music through your chosen path to identify any EQ adjustments that you may need, and in order, try to do those a) on The Katana, then b) the OUTPUT 2 EQ of the FM3..
2. The Katana is not a traditional combo guitar amp.. it is a modeling amp with a intended neutral response from the speaker, so you may well find that leaving power amp simulation and/or cab modeling either or both ON may give your more pleasing tones..
regards
