A sub should work out fine once you get it adjusted for the situation. Every room and setup is different, so you'll want to experiment with placement, gain levels, low pass levels, and polarity. I'd hesitate to go any higher than 80hz as a low pass set point ...that's about the range where most male vocals start. The higher you go, the more vocal range that the sub will be covering (usually not ideal), and the easier it will be to locate the sub as a source.
My subwoofer is set at 50hz to cover just the lowest octave or so. Its set at very low gain levels, and the polarity is reversed. It just augments the bass of the main woofers that run ~ 30hz to 80hz...it works really well in my situation.