Hi Jon, the EuVerb in the Gallery has a cutoff control in terms of Hz so it might be an easier one to control in this case. To get it:
Open a samples file in Kyma.
In the sidebar, disclose the Gallery section.
Check the Modifiers box.
Under Modifiers: Sources: Reverbs, open one of the examples whose name starts with the prefix 'EuVerb' followed by your sample name. !LPFCutoffEffect is in units of Hz.