The 'big knob' has nothing to do will USB 1/2 recording level output.
I have picked randomly one of the factory presets, in this case US Small Tweed (others give a similar result). Without any fiddling with levels I should get a reasonable output level from the USB 1/2 multi output directly into a DAW, in my case Cubase 8.5. In all cases I can only get about 50% of the volume needed to make a decent recording level. If I turn the Multi Output level up to + 12dbs or more I do get a reasonable recording level, which is ridiculous.
I have tested this with different DAW's and 3 Guitars, all give the same result. The only common factor here is the Helix USB output.
I can get over this problem by using the 1/4" Output into another Audio Interface (Steinberg UR22) but that defeats the useful function of the Helix having a direct USB recording output facility.