There was a lot more discussion of highpasses than I really thought necessary in the original F-20 thread. My opinion is that quite simply - asking a bandwidth-limited sub to perform outside of its intended bandwidth is a bad idea. At a minimum, distortion increases. In some cases, driver excursion increases.
Simple way around it? Only amplify the signal that the sub can reproduce.
While I am not going to argue the presence of recorded information well into the single digits (I know it is there, I have measured it) in some source material, you've got to decide where to get off the train. That means leaving something on the table, it is all about what represents an acceptable compromise. As you've chosen the F-20, I'd suggest that the appropriate station for you to get off the train is 20 Hz.
Seriously, while there is content below 20 Hz in some signals, and the driver will not likely damage itself trying to reproduce it at reasonable levels, the nature of a horn is such that it increases the distortion level to where distortion equals the fundamental within ~1/2 octave below the tuning point. As I am after low distortion reproduction of the original signal, I'd rather not have the speaker distorting unnecessarily. This is my personal preference and opinion, based on measurements I have collected.
How to go about it depends a lot on the rest of the equipment. Plate amps will often have a suitable highpass built in, several have been recommended in this and the original F-20 thread. Rack-mount or pro amps often do not, but there are suitable alternatives. I like the MiniDSP. No, it is not perfect, but it sure is cool. In addition to highpass, it provides a lot of equalization to help fix room problems.
If you're planning to run pro amps (which I'd strongly suggest), the Behringer iNuke DSP series (I'm not making this name up) offers a combination amp/crossover/EQ solution that may be perfect, but they are still vaporware - I have not actually seen one or read an end-user review yet. They are due to be released shortly. Unless something has changed recently, the DSP in the Crown XTi and Peavey IPR series of amps does not offer a low enough highpass for home theater use.
In a nutshell - I choose to highpass my subs that have limited bandwidth (to be honest, that includes nearly everything I have built in the last 25 years...). I typically do so with the filter in the plate amplifier or an outboard DSP solution. I place and phase the subs to minimize nulls in the response. Finally, I EQ the sub to correct room issues to the degree possible by shaving off the peaks in the response.
Measurements of your subs in your room are a fundamental part of this process.