I don’t use a USB connection but I still hear a hum.
Check your line cable for possible damage. Since your Blofeld is equipped with unbalanced line outputs, very long and/or badly shielded cables can cause interferences. Please make sure that you always use the shortest and best cables possible. If you use the Blofeld live it is possible that you may have to use a DI box (active or passive) that balances the line signal and thus eliminates any interferences or distortions. Please only use the line outputs. The headphone outputs are not meant for any balanced signals.