How do you use subwoofer phase control ?

Recently obtained 1986 era Duntech Thor powered large bench-sized subwoofers with outboard crossover box. No instructions about dialing in the phase somewhere between 0 to 180 degrees. What does one do to choose an optimum setting? Is one sub adjusted separately, and then later the second if allocating one sub per channel? May simply be using one of the subs equidistant between stereo Duntech wall mounted speakers as room size dictates due to massive "coffin" appearance of each sub. These subs specifically designed to complement these wall mounted speakers. Quite a contrast from the modern micro-sized subwoofers such as a Carver.
I set the phase by ear. My subs only have the option of 0 or 180. The option to set phase is only to eliminate the situation where the bass wave from the sub and the bass wave from the mains are cancelling each other out( peaks vs. valleys ). You ideally want both to be in sync.
in the phase of the subwoofers 0 to 180 degrees 0 degrees means its flowing with the music 180 means its changing its phase off set from the timing of the music. experment with the phase is not critical . sometimes changing the phase to 180 will sound better depending when the low frequincy hits you at your listing position. experment my man. happy listing.
Thanks to both responders for the explanation.
I need help on this one too. I have three MG1.6 speakers with three wall-mounted JBL 15" drivers for subwoofers. Crossover at the moment is 70Hz, 24 Db.
I have no problem getting flat response down to 20 Hz.
To adjust the SW phase, I set the warble tone generator to 70 Hz, so that both speakers are putting out sound, and I measure the sound level using a meter, while I vary the SW phase from 0 to 180. NO CHANGE!! I can't explain this. Can you?.
The best way is to take a SPL and set a level for the crossover region(like 80hz) Set the subs phase for maximum output at this level with the main speakers and it should be in closest phase. However, room anomalies can change this situation somewhat. I have found it is pretty easy to tell when they are in phase. Place your head between the mains and subs and listen. It shouldn't sound like there is a suckout.
Sounds like the crossover isn't where it is suppose to be. Try changing it up or down and see if you get a difference.
Bigtee..Thanks, but I know the X/O is at 70 Hz, and anyway the objective of having both speakers putting out sound is met even if the Hz isn't exactly 70 Hz.
I have one idea that I still need to try out. Perhaps the crossover (Audio Control Richter scale) is doing phase shift things around the X/O frequency. The test signal is a warble tone (range of frequencies) which is good for setting level but which may average out phase over the range of frequncy. I will drive both speakers full range, and see what happens when I diddle the SW phase. I will post results for your info.
Setting the sub x-over too high so that the sat/sub overlaps too much may make it easier to hear the changes in output when changing the phase. When you get the most output your sub is matching the sats. Then back down the sub x-over to its best blend. It helps to have someone listen out in the room as it can be tricky to judge when you are right on top of the sub.