I've just discovered a strange filter artifact in my KX3's filtering. Whilst fiddling with freedv, and adjusting the PBT high cut frequency with the rig in LSB mode (not DATA-A), as I adjust the PBT-II function downwards, a notch appears at 1500Hz, as soon as FL2 activates. I have the optional roofing filters fitted, as shipped from Waters and Stanton, and have performed no calibrations myself, yet.
Just to prove that its no artifact of freedv, the same effect can be seen in fldigi.
Update: Yes they have. It is explained here.