Old thread, but I just got my Helix floor unit, and wanted to add my observations, using Firmware 2.50. I'm using an empty preset--no amp model, no FX blocks, nothing. Plugged a sine wave generator into guitar input: 1KHz at 1v p-p. Then I connected my oscilloscope to the 1/4" mono output. The big Volume knob allows unity output (1v p-p) at about 1:30 o'clock on the dial. With Volume at maximum, the output is 2.5v p-p, or 250% gain.
I plan to use Helix as a stomp box (no amp modeling) in my cover band. So the question about unity gain is a good one--I'd want my guitar to sound identical plugged directly into my amp as it does when run through the Helix with all FX off. In factory default mode, this means setting the Volume knob to 1:30 o'clock.
In Global Settings > Ins/Outs, if I change 1/4" Outputs to "Instrument" instead of "Line", then output at max Volume knob setting IS unity, and is not boosted.