It's not super complicated. I basically just run the FM3 Out 2 directly into the FX return of the Katana (see picture).
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I'm using a TRS cable, but not sure that is necessary. As mentioned the patch I have built out splits the signal right before the outputs. I run to Out1 with a cab block, but Out2 goes straight out. I have a PEQ in the chain but haven't done much with it.
A couple of regarding set-up:
- In the Katana make sure whatever bank you are using has the FX loop after everything. I think you have to do this in the Boss Tone Studio. Unless you want to do something like use Katana FX (which I would not recommend).
- I've played around using different amp models in the Katana and it doesn't seem to matter, though there may be some settings which will impact this. Basically I think the Katana power section has a single color no matter what else is set.
- In the amp block of the FM3, I'd recommend setting the SS PWR Amp + Cab in the Output Mode section. This is the recommended setting from the manual for using a solid state power with traditional guitar cab.
- Play around with different Speaker Impedance curves in the FM3 amp block. I think the 2x12 Recto seemed to work the best for me.
- You'll likely want to play with PEQing to get the sound you want. I found the Katana speakers to be slightly darker and definitely mid-heavy (honky sounding) so you'll want to dial that to your liking with EQ.
I should also disclose that since posting that, I tried this same set up with a Seymour Duncan PowerStage into a 4x12 and got much better results so am going with that now, but the Katana option is definitely workable.