Dale's Random Ramblings ;-)

Menu 46 DSP High Pass filter Default at 100. Set that to 200 to 300 on the low-end

Menu 48 DSP Mic Equalizer

Press and hold in the [DSP] key for one second. This instantly activates Menu Mode No-048 [DSP MIC EQ].
Rotate the DIAL to select one of the following equalization choices: OFF: Microphone Equalization Off
LPF: High Cut (lower frequencies are emphasized)
HPF: Low Cut (higher frequencies are emphasized)
BOTH: High/Low Cut (mid-range frequencies are emphasized)
When you have made your selection, press and hold in the [FUNC] key for one second to save the new setting and exit to normal operation.

You also have menus 016 Carrier Balance -300 to +300 LSB Transmit and 018 Carrier Balance -300 to +300 USB Transmit Adjusting these controls will give you more low response or high response. As always listen to your test signal through headphones connected to a second receiver. You will hear the balance that will please you.

As always study the operation guide for further information.

Reference:
Heil Sound – All Things Yaesu
Audio processing and ALC in the FT-897D