Sub bass separation is a common core, basic idea about music production that is important.
Sub bass is the fundamental tone of the bass or the lowest single harmonic.
When the sub bass is not consistent or solid (wobbling all over the place), you want to look at what the sound wave looks like by itself. We want the sub bass to look like a regular solid sound wave. Sub bass separation is important to the mixing and mastering process; you want to squeeze every last inch of effect into the sound waves. Start by focusing on the main sound and remove the low end by high-passing the sound wave. You can do this by ear or by 150 Hz. Then, layer in a separate sound wave with the original sound wave. Using the Master EQ, you can see that being with the sub (together at the low end), despite all the crazy movement with the highs and mids, the low end remains static.
You can use the sub as a sound wave or use other kinds of waveforms to develop the sub bass. For example, you can distort a sound from the first harmonics in the solo or play an octave in the sub. Saturation is effective at adding the harmonic profile of the sound wave. The entirety of the low end contains the most powerful, loudest, and destructive elements of a mix. When you high pass everything, as described, you only have to focus on two things in the low end mix; the bass of the kick drum and the separate sub bass that was just created. This makes the process so much easier.
Some artists layer a single sine wave in their resampling, meaning sub bass separation is performed per sound. Similarly, you can high pass everything at 150 Hz and create a separate plug-in of a sine wave or take the sub from the source. You will have an unaffected original sub from the actual original plug-in. This is a big difference from the several manual subs versus using the source directly. You are mixing at the sound level and this is especially beneficial because it is a Reese sound that still has phase cancellation doing the pitching. Many plug-ins has this option, like Serum. Serum has a sub oscillator with direct output and this bypasses everything for you to engage your high pass. Serum allows you to change values to achieve the sound you want with different options, such as a sound wave, saturated sound wave, triangle whisk, square wave, and impulse.
Sub bass separation requires processing the sub with clean, unaffected sub frequencies to create unity. This is the solid bass; a clean and unrelated waveform. Processing this way is recommended for mixing and mastering.