I'm not naturally a 'phone operator and avoid playing with audio settings as much as possible. I prefer simple “communications quality” audio when I use SSB. As the FT-891 is intended to be my “phone mode radio” and I've done some initial adjustments to try and achieve a clean, basic communications-quality setup.
From what I can work out, the “bandwidth” settings of the EQ refer to Q - the smaller the number the lower the Q and the wider the affected bandwidth. I'm aiming for gentle shaping of the voice band to cut the low-end boomy bass and lift the high end “articulation” so I've set a low-Q on each pole of the EQ filter. I've set both the non-Compression and the Compression EQ settings to the same values - I want the audio to sound the same with and without the processor. The Mic's rear “tone” switch can be used to give a slight increase in low frequencies by switching → 1 if I ever feel the need.
Here are the settings I've got on the FT-891