Thanks, @phil_m for clarifying again. I'm aware of the different modes and I know how IRs work. And while I like the sound of them or the Helix cab-blocks when playing with headphones, I'm looking for a way more traditional sound and feel when the "amp" is sitting right next to me. And I really hope that I can get there when I get the whole gain staging worked out.
I did some more reading and testing and I also rewatched two videos (Sadites and Sterlacci dialling in PC+). The videos show completely different levels there, but both don't seem to use L6-Link, Sterlacci even levels only the PC+ with a full Helix preset (he adds +6dB to the 1/4" input from PC+). When I level everything up to the max, the Powercab sounds definitely better. But I still doubt that it was designed to need the maximum boost of +20dB in Helix to even begin feeling alive.