How do you high pass your main speakers?

Marchand XM446 balanced high pass filter.  
Scroll down to see the balanced version.  It is a passive filter and works well.

Here is another interesting option.

@k600r 

I personally believe you should set your crossover point by ear.

That practice has worked well for me.  Knowing the frequency response curve of my speakers does help as a starting point.  When I finish setting up my two subs, I subtly miss them when they are turned off.  A remote control to adjust the sub’s volume +/- 1,2,3 helps optimize for different listening levels and musical material.

@dbphd 

I am glad to hear that Charlie Hansen (RIP - another bicyclist lost too early due to a vehicle incident) approved of the passive balanced Marchand high-pass filter that I recommended earlier in this thread.  That filter seems to do no harm in my system.

The effectiveness of using a high-pass filter, and the best-suited cut-off frequency and slope, will be largely dependent on the main speakers.  Main speakers that do well handling high current and rolling off at their lowest frequencies without issue are possible candidates to be run full-range with a sub rolled in at an appropriate frequency.  Main speakers that do not do well at the lowest frequencies, and speakers where the bass driver(s) also handle midrange duties, are more likely to realize a sonic improvement by using a high-pass filter to limit the depth of the low frequencies that need to be handled by the mid-bass driver.

My Aerial LR5 speakers offer the advantages of being a 3-way design (where the low frequency drivers do not also handle the mid-range) and having bass drivers that extend low, down to around 40Hz.  They are also quite dynamic and designed to handle high power.  The designer, Michael Kelly, suggested running them full range and rolling the Aerial SW-12s in to fill in the lowest frequencies.  I have found that set-up to work well but when something else in my system (DAC, preamp, cables, etc.) results in a more bass-heavy presentation, then adding the Marchand high-pass filter (45Hz cut-off) between my preamp and amps does seem to help without causing a sonic penalty.   

@mapman 

In particular with small speakers, removing the two lowest octaves can greatly reduce the distortion, not just across the bass but well into the midrange, so the speakers sound more transparent with improved soundstage, and eliminating the bass also increases the speaker’s dynamic range a great deal. It’s a really life-changing experience.

Even if a speakers output drops it doesn’t mean the cone displacement does, quite the contrary, an effect which is made worse with ported speakers.

Well said.  I don't know about "life-changing" but your other points are spot on.  IME, the effect/benefit is greater with main speakers that don't go too deep or don't handle high power well.  I use the Marchand XM446 fully balanced high pass filter and because of the low frequency capabilities of my main speakers, I had them roll it off pretty low at 45Hz.  I roll the subs in about 40-45 Hz and the result sounds great.  Neither of their passive filters, the XM46SB-AA (single-ended) at $325 nor the XM446XLR-A (balanced) at $525, break the bank for a well-engineered high pass filter using high quality parts.  For folks seriously looking at this option, I suggest calling Marchand and getting what you need, rather than farting around with capacitors.

@erik_squires 

Your quote from Troels sounds as if he is discussing a 2-way speaker, where the midrange does double duty serving the bass too.  Why would the midrange be “pumping deep bass” on a 3-way speaker?

Not.all stand mounts are 2 or 2.5-way speakers but even with woofers dedicated to bass frequencies, there can be benefits of using a high-pass filter when using subs, such as keeping the LF drivers operating in the range where they are most linear and relieving the power amplifier of some back-EMF coming from the speaker drivers as presented in the quote from Troels that you shared.