With a sub amp power we are limited with the low cutoff : for example it is 40Hz with the Buttkicker BKA-1000N. With parametric equalizer in J River Media Center, we can make a low cutoff of 20 Hz for example.
Moreover, the most important is that with a sub amp it is impossible to choose wich signal to send to vibrators. We can send the subwoofer signal, but it is mixed with the bass of others channels. Integrated in J River Media Center, it would permit to choose to send the subwoofer signal and only it, without the bass of others channels.