It is true that it is good to match the roll offs in each direction.
A sealed box woofer will roll off at 12db per octave and having a sub woofer plate amp with the same roll off will match that, but more importantly is the wavelength that they are crossing and the distance that they are apart. In other words, how well they are in phase.
Now what if your sub has a 18db or 24db crossover slope? Then the ported box will allow the smaller woofer to play flat longer then fall with a sharper 18db per octave slope or greater matching the plate amp even better than gradual slopes.
The higher you cross them the shorter the wavelength. So they won't have to be to far apart to cancel one another out. A sub might be in phase with one speaker and near out of phase with the other due to distance. Variable phase controls on the sub amp doesn't help very much if the sub is a lot further away from one speaker than the other. Which then do you adjust it for?
If running them up very high then using a pair is a wise choice.
Still the lower that they are crossed the easier they will blend.
Al posted some cabinet prices here:
http://www.audiocircle.com/index.php?topic=29942.10