A few years back, I stumbled upon a
trick that’s infinitely more precise than
either of the above approaches, and is
blissfully simple: Just add a sine wave.
Ableton users may notice preset instruments
that include a macro knob labeled
“Body.” This knob mixes in a sine wave
either at the fundamental frequency or an
octave below. With this approach, we can
emphasize only the frequencies present in
our bass line, without introducing mud, distortion,
or unwanted overtones to the surrounding
frequency ranges.
Simply duplicate the entire bass track.
On the copied track, replace the original
synth with a single sine wave oscillator,
then mix copied and original tracks to taste.
Most soft synths include sines as a waveform
option, but if for some reason none of
yours do, a triangle wave will work. Though
it’s not quite as clean, adding a lowpass
filter to mute the triangle’s harmonics will
help it blend better.
Below are three examples using Propellerhead
Reason’s Subtractor synth,
and the process is really straightforward.
Use oscillator 1 as your primary waveform,
then use oscillator 2 to slowly add the
sine wave.
One note of caution: It’s easy to get
carried away with the sine wave approach
and add too much low-end to your bass
line. When in doubt, compare your mix to a
few commercial releases to ensure that
you’re bangin’, not boomy. A little goes a
long way!
Ex. 1. Here’s the initial patch: a single-oscillator sawtooth (oscillator
2 is turned off) with the filter cutoff about 75 percent open. 
Ex. 2. Now, we turn on the second oscillator, which is generating a
sine wave at the fundamental frequency only. For demonstration
purposes, the sine is mixed a tad loud in the audio example at
keyboardmag.com/how-to, so you can hear its effect. 
Ex. 3. For even more bottomy goodness, here’s the same patch with
the sine wave tuned one octave lower than in Example 2. Again, it’s
mixed in strongly for demonstration purposes.