There are definitely different approaches to this subject. There are just too many details to cover, but will go over some main ones. I fully agree with you Charles on the use of two subwoofers, but surprisingly disagree on some of the other information.
Always 2, 4, 6 or higher number of subwoofers. I use 6 myself. Going from 2 to 4 made an enormous difference. The step up to 6 was more academic in nature. These even numbers are of course to 1) better fight room modes and resonances in the listening space 2) lower the distortion by sharing the work between many identical moving parts.
There is a good reason for the THX-standard for a crossover to be set at 80 Hz. It is sort of a middle ground, but where the positives outweighs the negatives. Also I absolutely, 100%, am an advocate for using high pass filter for the main speakers (and low pass for the subwoofers). We should relieve the mains of the hard task to play deep bass, in order to lower the distortion as much as possible. Membrane and surround moving will undoubtedly cause the distortion to rise. 80 Hz is also suitable because at lower frequencies, it is very hard to locate the separate subwoofers, especially in a normal listening room. Also you need good drivers which are not distorting creating sound in frequencies higher up, which makes them easier to identify.
Of course you also need a crossover good (and steep!) enough for this. My active filter is 30/18 (low/high pass). The built in crossovers in surround receivers are usually not step enough is my experience.
I absolutely prefer a placement in front of the listener, close to the main speakers. 1/4 in of the room width is a good approximation that has worked very well for me. Mine are now standing as towers right next to my main speakers in this position. Another good placement is evenly spread out along the speaker wall. This will fight the room modes very effectively.
Most commercially available subwoofers, also insanely expensive ones, seem to prioritise one thing above all and that is a small size. It is such a shame, as it is near impossible to get true, good, deep bass in a small cavity. Still no matter price range, they seem to be as small as physically possible instead of letting the driver specification dictate the size.
As a reference, I have shared my own solution here previously: Showing (off) your Roon setup - description and photos
Temporary setup and will be fine-tuned in the coming months.