My band added a single Alto 12" sub to our (15") tops and the improvement in sound was very marked. I even DI the bass now so we can have a lower sound level on stage.
Going through the PA are vox, guitar, bass and keys. Very happy with the sound. We use the DSP 15 setting even though ours are the older version. Quite a weighty beast (even though active) but adds enormously to the sound.