TC-Helicon Harmony Singer

If you are having issues with feedback try

Set the Tone Style to Less Bright

To enable the 'Less Bright' Tone style, press and hold the 'Tone' button then press the footswitch. The tone button will blink to notify you it is engaged.

This may aid you in dealing with bright feedback issues.

To disable the 'Less Bright' Tone style, repeat the process.

Here is a link to a video outlining the process: http://www.tc-helicon.com/mic-mechanic-harmony-singer-firmware-update/

Note: If you cannot enable the Less Bright Style, you may need to install the firmware update.
Mic Mechanic and Harmony Singer Firmware Update