@tony1954 Great year! I'm 02/12. I have been crossing over to subwoofers since the Dahlquist LP1 was released in 1979. The LP 1 used an active 2nd order low pass filter and a passive 1st order high pass filter. They gave a kit of capacitors and you had to solder the right ones into the unit based on your main amp's input impedance. Fc = 1/2PiRC R is the input impedance of the amp and C is the capacitor in Farads which you had to select based on the crossover frequency you wanted. This is a first order or 6 dB/oct filter which is very slow. You can do this by putting the right sized cap in series with the input of your amp. I soldered them in all the time and I recommend this for those who have subs that only have low pass filters. You can get analog two way crossovers like the unit JL Audio makes but even better is digital bass management which allows you to time and phase align the subwoofer as well as allowing much steeper slopes without distortion and Room Control. These units are made by MiniDSP and dBx. There are also full feature digital preamps that include bass management from Trinnov, DEQX. Legacy, MiniDSP, and Anthem.
@ditusa's link is excellent and you should read it through.
In order to fine tune a subwoofer's crossover you need to be able to select both frequency and slope of both the high pass and low pass filters independently. My ancient TacT digital preamp gives you a selection of crossover frequencies from 20 to 300 Hz in 1 Hz intervals, 1st through 10th order and you can change them on the fly with the remote control! Today only the DEQX gives you this kind of flexibility. The goal here is to use the highest crossover frequency you can using the equipment you have. If you can hear the subwoofer, tell there is one in the system other than feeling the lowest notes then you still have work to do. The higher the crossover point the more likely it is you will hear the subwoofer. The slower the slope the more likely it is you will hear the subwoofer. Turn off the main speakers and listen to just the sub. If you can hear midrange coming out of it you have a problem. You have two choices. Lower the crossover point or speed up the slope. I use Sound Labs ESLs. I currently high pass the SLs at 120 Hz 6th order and low pass the subs at 110 Hz 8th order. Because removing the low bass from the main speakers lowers distortion I never recommend using a crossover lower than 80 Hz. If the main speaker is a full range driver like my SLs you do not want to go lower than 100 Hz.
The next problem is time and phase alignment. If you do not have the ability to adjust this digitally the best starting place is to keep all your speakers and subwoofers equidistant from the listening position. Place you two subwoofers at intervals between the main speakers right up against the wall. Using a tape measure measure the distance between the subs and your listening position. Now adjust your main speakers so they are the same distance away. If you want them farther off the wall you will have to move them out laterally. If you can not make this set up sound right then the smartest way to proceed is by getting a calibrated USB microphone and see what is happening. You can then adjust the speaker positioning to minimize group delays. Or you can get a Digital preamp that will come with a measurement mic and let the computer do everything for you. The other beauty about digital preamps is that they usually provide digital EQ capability which allows you to make your system sound exactly as you want it to without adding any distortion. You also do not need a separate DAC as they are included. The MiniDSP HSD is only $1,200.00. At this moment I think the soon to be released DEQX Pre 8 and Pre 4 are going to be best. They have not released the prices yet. I am going to be Beta testing the Pre 8 shortly.