I know firsthand that water softeners take out GH, but don't touch KH. The only thing I can think of is to dilute your water with safe water that tests lower in KH.
Place some peat in a mesh bag, container in your filter. This will tan your water a bit, though. You can also dilute with RO water or any water with a lower KH, as said.
Acids also naturally lower your KH and will drop pH accordingly.