.. and what the enclosure for the sub is tuned for.
General rule of thumb, for size of box vs freq. response, is; the larger the box, the lower the frequency response... but now, even very small enclosures can be tuned and built for extremely low Hz ranges... so that rule applies less and less every day.
I would look more at your remaining speakers [full range] to determine the best range for your sub to play. You dont want too much 'overlap' with the freq ranges for the different speakers.. otherwise you end up with very "boom-y" mid ranges, as the sub and the front speakers are playing too much of the same notes.
Hope that helps some.. Does your receiver have Xovers for the other speakers as well? If it does..and if your front speakers can handle some decent bass.. I might recommend ~70-75Hz for the LPF/HPF point for sub and front speakers.
- listen for any slop coming from the speakers as you are adjusting, and use a variety of music/movies to tune with.