TC-Helicon Harmony G to Compact
Connecting the TC-Helicon Harmony G microphone output to the L1® Compact Channel 1.
The functions of the TC-Helicon Harmony G Tone feature overlaps the functionality of the Compact Channel 1 Microphone Preset. As a result you may experience a greater tendency toward microphone feedback when using the Tone feature. You can overcome this by turning off the Tone feature in the TC-Helicon Harmony G.
Here's how you can disable the Tone functions in the TC-Helicon Harmony G: Press the Tone Button to turn it off. (light out)
From page 16 of the TC-Helicon Harmony G manual