Hi Greg,
I started with 2 subs and didn't think it properly blended with the sound in the room. When I purchased the other 2, I started with one of them in my listening chair and did the sub crawl around the room. The 4 best places were approximately the center of the front, back and side walls. At about 14" off the wall.
They were like that for a number of years while I was sending them their signal from my preamp. I sent right and left signals to the closest sub and then a mono signal to the others.
In the last year I decided to go to sending their signal to them via speaker cable from the power amps. With that I moved the sub that was at the back of the room to the front and located them partially behind the main speakers.
From there I ran a cable to the left and right side wall subs ending up with 2 left and 2 right subs.
I had previously found that sending the same signal to the subs as the main speakers received made for better integration and adjustments on the subs controls when I had Vandersteen subs in a previous system.
The sub adjustments is quite time consuming with 4 of them though.
I started with 0 phase and one at a time adjusted them to 90 degrees out of phase, then 180 and finally 270. Which ever sounds best I would move to the next sub and settings. When you throw in the crossover point, then ability to make slight + - db change above and below 25Htz and then volume settings it takes a while to get things where you like the sound.
The last time I had my EE friend check their response with his equipment there was a slight bump in the lower frequency. After an hour or so changing settings we discovered the settings were fine but the left sub was 4" farther away from the left main than the right side. We moved it and ended up with an amazing smooth graph. We were both quite surprised that the 4" made that big a difference.
I hope that answers your question. If not, feel free to ask and hopefully I can answer the questions.