Using Dual Output Monoblocks to Run a Passive Sub

I have a pair of monoblocks I am currently using to run monitors. I've considered adding a sub to my set up but do not wish to compromise on the quality of the amp in an active sub. My setup may be somewhat unique, because the monoblocks have two sets of binding posts, each of which receives power and a signal (though I suppose this is similar to what one could do with a stereo amp and two sets of speaker outputs).

This means that I can run a second set of speaker cables to two passive subs with a crossover. Just curious about recommendations for reasonably priced setups or if there are any issues regarding running the monitors in full range as well as the subs with a cross.