@pr2-eugin: That worked nicely, thanks.
After some fiddling, I've managed to include the other three combo boxes as follows:
Me.Filter = "[Asset Group] = '" & Me.cmbFilter1 & "' And [Calibrated?] = '" & Me.cmbFilter2 & "' And [PAT Tested?] = '" & Me.cmbFilter3 & "' And [Location] = '" &...