Why Add Harmonics to Sub Bass?
Small speakers, like those found in a notebook computer or smartphone cannot properly represent very low frequency material. Programme material that features a lot of sub bass instrumentation will most likely sound thin and sparse when referenced on these types of speakers. Adding additional harmonics to the sub bass parts will help.
Introduce a small amount of distortion using an aux send on your DAW or mixing desk to add related frequency content above the fundamental sub bass tone. Doing this will appear to make the original sub bass part audible on tiny speakers by tricking your ears into hearing something that may not actually be present. Remember that subtlety is key, so even percent or two of distortion will help.
Add an equalizer or filter after the distortion and experiment with filtering out the high, low or mid frequencies of the distorted signal to add interesting sonic character. Different forms of distortion, like analogue distortion from transistors, tape and vacuum tubes or even digital clipping will also yield different and interesting harmonic texture. Choose the one that best suits your programme material.