Eh hem!...Subwoofers... What do ya know?

Subwoofers are a thing.  A thing to love.  A thing to avoid.  A misunderstood thing.  

What are your opinions on subwoofers?  What did you learn and how did you learn it? 

Theres no magic to placing your mains . Rule of thumb is placing at eye height and the distance between them is how far either is from your head . 60 degree triangle . I’m not quite sure what you are asking but the soundstage is the biggest part . Make sure the set up is in the center of a wall . Its all about YOUR ears . Everyones ears are different. All anyone can recommend is better math.  Other wise ear buds wouldnt have adapters for different shape ears. Poor symmetry   makes premium equipment sound midfi . We were only talking about subs and the various methods of use. 
@davekayc  I'm fighting a bass null at the sitting position so I wondered about, having the mains spread further apart but toed in at a steeper angle to limit low frequency waves going straight at the wall and being reflected back. Basically having the very toed on speakers sending low frequency waves at the rear wall at a steep angle vs nearly head on to negate the null.

Unless I move at least 38% of the way into the room (better at 50%) I sit in a mid bass null. I always thought the speakers were bass shy but the sub I've been messing with has the same issue so its clearly the room and not speakers. If I mive 3ft to the right of the couch/sitting position which puts me in front of the hallway entrance the sub is delivering THUNDER. Move back to the sitting position and its a little warmer at best.
Turn the crossover all the way down . To 50 or 63 depending what you have. Then try as i suggested which i learned from duke and tim. Put the sub in your chair. Now listen all over your room . You may have to crawl on the floor to find the hot spot . I used a phone app with the spectrum on it. And listen . There are spots all over in your room and they may not be where you think. Now put you sub at one of these spots preferrably the loudest spot . And turn your sub down. It will be far more efficient now. 
toe in helps keep reflections off the side walls . Improves clairity and stage. Dont toe in so far that they aim in front of you . Most you want is at you .
my mains i set up with a building lazer. Centered , same angle and 40” from tweeter to rear wall . My focus point 20ft behind me . I dont have side reflection because my side walls are also 20ft away . My small living room is normal and my speakers aim directly at me. No sub required. 
gochurchgo, here are some starting point guidlines for a speaker placement and a youtube for the Crawl Test subwoofer placement. Since not every room has four corners, mapping out your rooms bass modes (where bass is louder) can be very usefull.

Not every speaker system was designed to be toed in.