How do I set the crossover for 2 channel listening on my HT system?

Your head unit is sending a low level signal @ what ever you set it at and normally that roll off is the same as the roll on of the next crossover point. No different than any crossover. IF they lap at the same frequency and have a vey gradual slope there can be all kinds of bloating. 

If you set your sub to a lower point the sub plate (whatever you have) will play lower but your signal is limited by how you're set up. If you would have set the subs in the head unit to "Large Speakers" the subs will be controlled just by the plate.

My subs are set at 4th order 24db but the slope from the mid bass coupler columns is at 48db. A VERY steep slope. Because of that alone there is little bloat and almost no combing or suck-out.

That is what you want to try to do, but remember 50% of great sound is the room, know your room. Ground up approach. Room, Bass, then all the rest, not the other way around. With todays systems you have to work to get bad sound to tell the truth.

Regards

I understand a subwoofer crossover should roughly match a speaker’s low frequency capability,

You want it to blend in with the low pass of the speaker in the room, which could be a lot lower than 49 Hz.  As I've written elsewhere, that's a complicated place and measurement is essential.  In addition, if you can use a high pass filter to reduce the bass to your speakers you will get lower distortion and higher dynamic range, and that means raising up the crossover point to 80 Hz or so. 

You do not say which Denon receiver you have but I would think that the crossover function in the Denon would contain both low and high filter capabilities so both your mains and sub will be crossing over at the same frequency. The low pass filter on your sub is not needed with type of set-up.

I would set the crossover frequency in the Denon somewhere between 60 and 80 hz and set the low pass filter on the sub at the highest frequency the filter allows.

Thanks to all. To oldhvymec4: The front speakers are indeed set to Large in the speaker set up of the Denon AVR-2807. I thus understand the crossover setting on the Denon is irrelevant and the low pass filter setting in the M&K will control. On the other hand audiorusty appears to say the low pass filter on the M&K is irrelevant-- yet it is suggested I set the low pass on the M&K to its highest available, which in this case is 100Hz.  These seem contradictory.

Eric_squires: The 49Hz figure cited is the manufacturers spec for the 805S. I haven't any instrumentation to determine precisely. As indicated that figure is ±3dB and the M&K rating is ±6dB so there is a fractional difference where the blended frequency should be which I also cannot calculate and thus have used the speakers 49Hz as a "compromize". 

Thanks to all. To oldhvymec4: The front speakers are indeed set to Large in the speaker set up of the Denon AVR-2807. I thus understand the crossover setting on the Denon is irrelevant and the low pass filter setting in the M&K will control. On the other hand audiorusty appears to say the low pass filter on the M&K is irrelevant-- yet it is suggested I set the low pass on the M&K to its highest available, which in this case is 100Hz. These seem contradictory.

The large/small setting in the Denon will control whether or not any bass is routed out the subwoofer output. The crossover setting in the Denon is irrelevant for the FL/FR speakers set to Large, but still relevant for any other speaker set to Small -- center, surrounds, etc.  Note that crossover settings do not apply to the LFE channel as that typically covers a frequency range of 3Hz to 120Hz based on the source mix.  Since you are using bass management in the Denon, you should set the low pass on the M&K to the highest setting, essentially disabling it. You don’t want to use both.