I agree that a sub is a less than perfect solution. I cheated by using identical amps and preamps, respectively for my mains and some good woofers for subs. I have complete control of how much bass I use without any crossover distortion. After many premade crossovers, after learning how to design them myself, and after years of matching subs, I finally tried dual, identical electronics and three inch thick foam for high frequency roll off for my subs, try it. you'll like it.
Experiment with towels first, a layer at a time. You will be amazed how much better bass can be with no crossovers and a separate volume control that does not change the amount of power by frequency, like a bass control.