Disclaimers:
I do not own either NI Komplete Audio 6 or Studio One.
I own Scarlett 18i20 and Reaper.
You'll need to translate to your device. I looked for the manual for it, but apparently it's a state secret.
The easiest way to re-amp with Helix is to spend $99 on Native, that's how I do it.
I decided to try to do it the way you're doing it because I know it can be done and wanted to see if I could get it to work. I'm crazy that way, and if the results of my struggle with this help you out, cool.
Notes on the Scarlett 18i20:
The Scarlett has 10 hardware inputs. Inputs 9/10 are the S/PDIF.
It has 12 hardware outputs. 11/12 are the S/PDIF.
The 18i20 uses the same mixer SW as the 6i6, and I imagine the I/O Channel #s would be relative. 8 HW Inputs makes S/PDIF Channels 7/8; 6HW Outputs makes S/PDIF Channels 5/6. The 6i6 is similar to your device, but YMMV.
First Steps - Helix
Recording Preset
Two paths, one effected, one clean, both Inputs set to Guitar, both Outputs set to Digital, one Output panned hard left (S/PDIF 1), one panned hard right (S/PDIF 2). Alternately, you could have a big preset with 1A into 1B, then split off 2B with Guitar Input and your second Digital Output.
Re-amping Preset
Input S/PDIF, Output Digital
DAW - Reaper
Track 1 Effected - Input S/PDIF 1, Output Master.
Track 2 Clean (DI) - Input S/PDIF 2, Output S/PDIF (Outputs 11/12) and Master
Track 3 Re-amp - Input S/PDIF 1/2, Output Master
Master Output 1/2
Mixer/Audio Interface - Scarlett 18i20
Here's where the complications ensue.
In order to get this to work, I have my Monitor outputs set to Software Playback 1/2. That's the Master Out in Reaper.
***S/PDIF Outputs 1/2 set to Software Playback 11/12.***
***MUTE this Channel during initial recording to prevent digital feedback (NASTY!).
UN-MUTE to enable re-amping.
I think that's it!
BUT....it's getting late, time to sleep. Hopefully, I won't re-read this over coffee tomorrow and go "OH NO! If he does that all the magic smoke will escape and he'll be very angry!"
Seriously though, keep your monitor /headphone volume low until you've got the bugs worked out. I wasn't joking about that digital feedback being NASTY!