A single monoblock handles a single channel; stereo has two channels. That's why you need two monoblocks for stereo, in a regular setup.
You might say that stereo splits up the music into left and right.
An active crossover like the ones that Marchand makes splits the music into high frequencies and low frequencies.
So you are trying to split the music in four slices. You are going to need four monoblocks to handle that.
There is one exception: you can play the low frequencies in mono. You sum the low frequencies of the left and right channel into a single mono channel. This actually works below 100 hz or so (probably 80 hz if we get picky about it) because stereo information tends to get washed out by then; the reason, basically, is that the wavelength of an 80hz sound wave is quite a bit longer than the separation between your speakers. This reasoning becomes more dominant as the frequencies get lowered. You may still prefer to have two (or more) subwoofers, but it will not be because the stereo effect suffers, rather because of low-frequency sound propagation through the room; but at least you can begin with one subwoofer and add more later as you see fit.
btw I recommend that you talk directly to Phil Marchand about any of this, as he is knowledgeable, straightforward and honest. If he tells you you need something for your setup, you most probably do. :)
