If you don't ever change the fader, then it's no big deal. However, if I were to set up a similar system (I've been far too lazy for far too long now), I might use a 2-way crossover on either the front or the rear pair of RCAs. The outputs are "high pass" for the speaker amp, and "low pass" for the sub amp.

A more likely solution, for me, would be to use a speaker amp that includes a low pass output to connect to the sub amp.