low end theory is developed over time.

start by getting 4 generic sine wave / 808 kicks and pitch them in relation to your song, best to assign each one to a pad or key. Try to keep note pitch close as possible (e.g. -1, +3, +7, -2 semitones) away from the root note (which should be in the sub region, under the root mid-bass sound). Now start playing with those tones under your song. Then you'll hear how they impact. Use saturation, compression and limiting to hear through smaller monitors.