This is a highly recommended install that fixes a bug in the Amp block that would cause lack of bass response when switching between certain models.
Mark I/II:
www.fractalaudio.com/downloads/firmware-presets/axe-fx-2/18.0/axefx2_18p04.zip
XL:
www.fractalaudio.com/downloads/firmware-presets/axe-fx-2/18.0/axefx2_xl_18p04.zip
XL+:
www.fractalaudio.com/downloads/firmware-presets/axe-fx-2/18.0/axefx2_xlplus_18p04.zip
Mark I/II:
www.fractalaudio.com/downloads/firmware-presets/axe-fx-2/18.0/axefx2_18p04.zip
XL:
www.fractalaudio.com/downloads/firmware-presets/axe-fx-2/18.0/axefx2_xl_18p04.zip
XL+:
www.fractalaudio.com/downloads/firmware-presets/axe-fx-2/18.0/axefx2_xlplus_18p04.zip