Helix XLR output to tube amp volume level

[MRP5G8]

If I want to use the Helix XLR output to a tube amplifier, would it be best to select "mic level" and increase the send volume or select line level and decrease the send volume?

 

On the same topic what dB level is "mic level" for the Helix?

 

For full context I want to send a balanced signal to a Radial StageBug SB-6 line isolator, then a short unbalanced patch cable to the tube amp (off stage).  The amp will be fed into a UA Ox, then back to the Helix via a coax SPDIF cable.  I'll be using the line isolator to help breakup a ground loop when using the Helix in 4cm.  I'll be using the Helix 1/4" outputs into a TS-XLR DI box (Walrus Canvas Stereo) to the FOH.

 

Thanks!!!