In the Radio settings tab, there is a highpass filter setting (Codec inp HPF Hz). You should set that to its lowest value, 82 Hz, and do this on both the Radio and Control end so that you can send and receive at the lowest possible frequencies. Beyond that, I don't know what else may limit low frequency response.
Also, you should be using one of the Linear audio CODECs (set in the Audio Quality item). Linear 16 bits 8 Khz or 16 kHz are best. The ones with compression will add distortion that WSJT does not like.
73,
Gary NA6O