Is what's described in here and in this PDF document what you have?
(Looks like the RF Punch 800.2 has the same filter switch/crossover frequency setting dial too)

Look at page 12 (you'll probably need to zoom in somewhat) and there's a switch
HP/FULL/LP (also see attachment). Make sure it's in the HP setting (HighPass, will only let higher ferquencys through) Then use the crossover frequency dial to set how low the HP filter will go.