The best reference I've seen for subwoofer placement is a book called "Sound Reproduction" by Floyd Toole.
Dr. Toole is either retired or deceased (or both), but was chief scientist for Harman and widely-recognized as one of the preeminent authorities on speaker design. I like this book because it treats the room and speakers as two halves of a single system, and thus deals more with acoustics than with speaker and speaker enclosure design,
Low frequencies are indeed relatively omnidirectional, that's not a myth. Why, then, do mastering studios often have two subwoofers? It has to do with their long wavelengths, which means most things in a room are physically too small to interfere with them (including the speaker enclosure itself.) If, for example, you were to take SPL readings around a subwoofer that was sitting outside in an open field and elevated off the ground, the readings you'd get for 360 degrees around the speaker would be surprisingly consistent.
Of course, nobody listens to music out in an open field. The point is that only very big things can get in the way of low frequencies. In a room, there are things big enough to interact with low frequencies, namely the floor, walls and ceiling. They are referred to as "boundaries" because they impact all frequencies. And they are the reason it matters where you place your subwoofer. It matters a lot, but it has nothing to do with directionality.
Boundaries reflect low frequencies and send them colliding back into themselves. If they happen to collide in phase, that's constructive interference that will result in a large volume increase at that point in space. Conversely, if they hit 180 degrees out of phase, cancellation will occur and that frequency will essentially disappear at that specific point in space. Finding the best spot for a sub is very important, and that spot will probably not be where you think it ought to be.
Oh, and crossovers are technically either low-pass or high-pass filters, depending on whether it's feeding a woofer or a tweeter. Most subs have both.
Conventional passive filters are single-pole, meaning 6 dB per octave. Active filters can be designed as steep as you like, although most will be 6, 12 or 24 dB/octave.
Crossover frequencies are typically between 70 and 150 Hz. However, that value will be determined by the characteristics of both the sub and your main speakers. There'd be no point in setting the crossover to 60 Hz if your mains only go down to 80 Hz. Also keep in mind that these filters have a gentle slope, so if your mains are 6 dB down at 60 Hz then 60 Hz would be too low a crossover frequency. You want to choose a frequency that's well above the point where your mains wimp out.