I am another church guitar player with noisy Helix XLR woes. For me at least, this is not a phantom power issue. My church uses a Midas Verona 560 pro board with individual phantom power switches on each channel. Phantom power is turned off for me, yet I still get loud hiss with the Helix XLR outputs. On the very off-chance the board is sending phantom power even though it says it isn't, I went ahead and bought one of the phantom power blockers mentioned in earlier XLR hiss threads on this forum (Sescom SES-IL-PPB XLR). I still get loud hiss even with the blocker in place. To recap my findings:
Hiss even with XLR phantom power blocker installed
Hiss regardless of Line or Mic level output
Hiss regardless of ground lift setting on the Helix
Hiss even when volume is turned all the way down, simply having the Helix on creates noise
My only recourse so far has been to abandon XLR and use a DI box with the 1/4" outs. Something ain't right about this Helix.