I have owned the servo 15/x-30 combo for a number of years and discovered that the active crossover does take something out of the high pass signal. It is an excellent sub crossover but leaves something to be desired for the highs.
Ecclectique hit it right on the head with using one set of outputs for the mains, and the 2nd set for the x-30/servo 15.
If not two outputs then the tape monitor output will work. This is what I am personally doing at the moment with good results. The x-30 with its infinite phase adjustment, crossover point, and gain control makes it very easy to blend the sub into a room (provided you dont stick it in a corner, avoid the common fractional cancellation points, etc).