Yes, I have tested having only my Helix floor connected to the Scarlett 18i20 using S/PDIF.
(I should mention I have the generation 3 of the 18i20.)
This worked fairly well with the 18i20 and Helix set to 44.1 kHz with the clock on the 18i20 set to S/PDIF.
However, I did still get some drop outs and lock synchronization problems.
It would consistently occur when using the Helix 6 button looper, start/stop would often trigger it.
After the help you and "rd2rk" provided above, here is what I've done so far...
(...Getting away from the S/PDIF connection and using USB for both the 18u20 and Helix.)
I installed the "Line 6 Helix Driver2".
I set up an aggregate USB device on my Mac…using Apple’s website documentation here:
https://support.apple.com/en-ca/HT202000
With the aggregate USB device set up, I changed my Helix back to use the default 48 kHz and also tried 44.1 kHz… both worked fine.
The aggregated USB device is going to be incredibly useful in my DAW (Reaper).
After testing this a bit, I disconnected the Helix from my Mac and reconnected it by S/PDIF to the 18i20, set to 48 kHz with the 18i20 set as “Internal”.
So far, this appears to be working well.
Thanks for the help!