See below my communications with Line6 support. It appears that nothing can be done
I just purchased a Relay G10 and I get sound with my Larrivee D-05 with Fishman under saddle pickup. Unfortunately there is also a really high pitched noise (almost like a fax machine) along with the guitar sound. I have tried the adapter 1/4 m to 1/4 female but the high pitch still remains. Please help John, I'm sorry you're having a functionality issue with your Relay G10. We cannot guarantee 100% compatibility with every guitar on the market due to varying input jack technology and configuration. If you already tried the adapter work-around to no avail, unfortunately that guitar has an insurmountable incompatibility. Let me know if you have any other questions. Kind regards, Jason-Line 6 support John, Thanks for your reply. There is no comprehensive list of the guitars with incompatibility to the Relay G10. Let me know if you have any further questions I can help with. Kind regards, Jason-Line 6 support