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

My system consists of a Denon HT receiver powering a pair of B&W 805S speakers (and, unrelated to this question, B&W center channel and surround speakers) with a vintage M&K MX-350 THX subwoofer fed through the receiver’s LFE output via single subwoofer cable. In 2- channel mode the receiver is set for L/R front and subwoofer play only, using the Denon’s “Direct/Stereo” play mode. The signal is fed to the receiver via RCA interconnects from a stand-alone streamer (Bluesound Node 2i) and DAC (Topping D90).

The 805S speakers have an advertised frequency range of 49Hz–22kHz ±3dB. The subwoofer has a variable low pass filter adjustment ranging from 40 to 100Hz. The receiver’s speaker set-up settings allow a crossover setting in steps from 40 to 250Hz. The subwoofer’s phase adjustment has been properly set.

I understand a subwoofer crossover should roughly match a speaker’s low frequency capability, i.e., in my situation the M&K low pass should be set at approximately 49Hz. (I say roughly because the subwoofer crossover frequency is listed at +-6db whereas the speakers is at +-3db). But what effect is the crossover setting on the receiver having and where should it be set for 2-channel listening in my situation? Is the HT receiver’s subwoofer crossover setting controlling the signal to the subwoofer and either cancelling out or somehow multiplying the subwoofer’s independent crossover setting.