Bass designMixingBuild Better Low End
Phones and laptops have no sub driver. If your 808 only exists below 60 Hz, most of your audience hears nothing at all.
An 808 is an instrument, a sub is a layer. Running both puts two notes at one frequency, and phase decides what the listener actually gets.
An 808 is played and a sub is placed. The 808 has an attack, a pitch you follow, a tail and whatever character you gave it, and a listener could hum the part back. A sub has one job: put the fundamental under the arrangement and draw no attention while doing it. Most tracks want one of them and no more. Adding the second is a real decision rather than a safety net, because two elements sitting in the same register hand you a phase relationship you did not previously have to manage.
The distinction is not frequency, it is behaviour. An 808 is a note event with an onset, a decay and often a glide, and every one of those things is information the listener tracks. A sub is a sustained tone with no onset worth hearing, no character and no independent movement. That is why layering a second 808 under an 808 rarely improves anything: you added a second instrument, not a foundation, and now two parts are competing to be the one the ear follows.
Use the behaviour as the test. If the element you are adding has an attack, a tone you would describe with an adjective, or a rhythm of its own, it is an instrument and it belongs somewhere other than under the bass you already have. If it has none of those and only holds the root, it is a layer, and a layer is allowed to be inaudible alone. It does its job when you notice its absence, not its presence.
Two tones at the same frequency do not simply get louder together. They sum according to phase: aligned they reinforce, opposed they cancel, and everything between those is partial. Because the 808 and the sub are triggered by separate oscillators with their own start conditions, the phase relationship at the moment of each note can differ from bar to bar. That is why a doubled low end sounds heavy on one hit and hollow on the next, and why no fader move fixes it.
Small tuning differences turn into level movement rather than into harmony. If the sub sits a few cents from the 808, the two beat against each other at the difference between their frequencies, and down there a slow beat is felt as the low end breathing. The same interference that gives a reese its movement is a fault when it happens under a bassline, for exactly the reason it is a feature there.
One of the two carries the root and the other stays above it. Usually that is the sub below and the 808 above, because the 808 is the one with character to lose. Make the choice explicitly: when both elements own the fundamental you have built the cancellation into the arrangement. Two instances of one engine covering sub and 808 synthesis at least keep both decisions in the same controls.
Balance the two by band, not by level. High pass the 808 above the crossover point and low pass the sub below it, so they overlap as little as possible and there is almost nothing left to cancel. Balancing with faders instead leaves both elements present across the same range, and the phase problem stays exactly where it was.
Both notes should start on the same sample. A few milliseconds of offset between two low frequency tones is a large fraction of a cycle, which turns a doubling into a partial cancellation. Zoom in on the waveforms rather than trusting the grid, especially if either element passes through a plugin that adds latency.
A tracking sub follows the 808 note for note, which keeps the harmony correct and maximises the phase risk because both elements are always on the same pitch. A held sub sits on the root of the key and never moves, which removes the interference and demands that the harmony tolerate a pedal underneath it.

PLUNGE covers sub, 808 and reese synthesis in one engine, so whichever element you build, the low end comes out of the same signal path.
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