Hi fedocable, I don't know anything about stricter books, but I've set a bunch of subs over the years. Bass does become directional between 80 and 100hz. There is no absolute answer to your questions. Room nodes and woofer placement cause or correct most problems with bass. I've never heard of a normal procedure of placing a sub an octave above the rolloff of the main woofer.
As for a single or double woofer? Double woofers are harder to place and if a single woofer is done correctly, a double woofer won't help much, but if you are trying to achieve the ultimate, a second woofer takes you the rest of the way there.
I would start with a single woofer, I'd cross it around 100hz, placing it away from the corners, between speakers would be good, get your gain as close as possible for a smooth transition. You will get very close to getting the woofer right or you will find a bass peak somewhere that won't go away. Either way, once your gains are good, I'd then take the crossover down to 60hz and see how the blend is. Depending on the room, you could still be dealing with a bass peak and may need to cross even lower. My guess is that it will be decent. Start dialing in frequency and phasing until you are a happy camper. If there are bass peaks that won't go away no matter what, see if you have a phase dial and start working on phase, if you still have peaks, you'll need some sort of bass traps in the room to absorb unwanted nodes. I hope this helps, Tim
