Javachip is certainly correct about the localizability of the sub when it is asked to perform much above 80Hz but that was not the problem asked about.
In fact, I see more problems in lowering the crossover from 200Hz to 80Hz, more than an octave, if the main speakers really are incapable of low frequency output. Doing so will force them to reproduce, with requisite power, an octave or more below their abilities. Since we do not know what they are, we cannot be certain but I would proceed with caution.
IMHO, better a localizable sub than blown speakers. The solutions include repositioning of the sub close to the main speakers and/or replacement of the main speakers with more competent ones.
Kal
In fact, I see more problems in lowering the crossover from 200Hz to 80Hz, more than an octave, if the main speakers really are incapable of low frequency output. Doing so will force them to reproduce, with requisite power, an octave or more below their abilities. Since we do not know what they are, we cannot be certain but I would proceed with caution.
IMHO, better a localizable sub than blown speakers. The solutions include repositioning of the sub close to the main speakers and/or replacement of the main speakers with more competent ones.
Kal