“601 mono blocks with new Klipshorns mk6" wow talk about over kill. you may not be getting the bass you expect due to never getting that amp out of idle.
Regardless, after years of trying to find a sub for LaScala’s I would recommend a servo sub or at a min a sealed design. This is to keep up with the horn loaded bass that’s much faster than most subwoofers out there. Integration will be the larges issue.
Also assuming you have the speakers in corners.