Have made several pairs trying different XO configs. My conclusion:
For moderate SPL's and a small enclosure, a simple two-way using 3rd order filter works well, but don't expect much bass from a single P13 in a sealed (fc=110hz)box.
For higher SPL's, consider an MTM (along lines of Ariel/ME2). A ported box (Fc=75Hz) or X-line will bring up bass response.