I should clarify a bit. IMHO, the greatest benefit you'll get from a(n) (optimally installed) sub occurs ABOVE 70hz. This benefit is the ability to address the quarter wave null and the "hump" that so often occurs about half an octave to a full octave above that.
That's why I highlighted Drew's observation. His crossover scheme in point #2 may work well for the 1/4 wave null, but there is another approach to the issue, too. You may find it easier to sharply low cut your main speakers and cross to your sub above the 1/4 wave frequency. You can then address the problem effectively by optimizing the sub's placement in the room.
Stereo subs crossed even higher (app. 120hz to 140 hz) will allow you to address the "thickness" that often occurs in this range. BTW, "bassbusters" or other hemholtz devices can also be used to deal with this issue. If you don't want to go that route, IME, you can usually place stereo subs such that imaging is unaffected despite the high x-over point and frequency response will much, much smoother through the most troublesome ranges.
Good Luck
Marty
