If the manual suggest 90, I would start there. A lot of people say that a good xover point would be a full octave above the F3 (in this case, that would mean 100hz for xover point).
How high you can set the sub will be a function of how close it is to the speakers. Ideally, you would have two subs, one sub near each, particularly if you prefer a higher xover.
For bass mgmt, the Behringer products get the most talk here, whether the DCX or CX. They will give you the most flexibility for the dollar, but they will be more intimidating to use, at least at first.
Doh, you say receiver; an external xover normally would require you to buy separate amplification. Hm, since you say Maggies, maybe you do already?
For the time being, you can use the xover built into the subwoofer itself. It will most likely be lowpass only (so the speakers are still getting the full range), but perhaps after playing xover point, phase, slope, etc, you can get it to sound just dandy. Just run speaker wires into sub, then out again to respective speakers.
