I have almost sorted out what is going on with Helix input and output levels by my and my friends research. The professional standard for line level audio signals is +4dBu.  If we assume 15dB headroom to full scale digital level (0dBFS) is the standard Line 6 has adopted then Helix has three level settings: Line level:  +19dBu full scale (+4dBu reference with 15dB of headroom) Instrument level:  +11dBu full scale (-4dBu reference) Mic level: +8dBu (-7dBu reference)