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Struggling to explain .....
polyphonic bass "muddys up the mix" to the best of my knowledge even though it sounds better in singularity
or something
first half of the example is polyphonic; second half is monophonic everything is the same otherwise
A bass instrument is typically played in single notes (i.e. monophonic) with the rare exception of playing higher harmonic notes on rare occasion as a quick filler....
Mixing low frequencies in a musical sense will not sound natural and will kill a mix as you have correctly said....
Although SOMETIMES you can do cool things with a second layer and a highpass filter to give a bland bass line some extra pzzazz... use taste ;)
You need open voicings for chords in the low registers, to keep them from sounding muddy. That's why the bass strings on a Chapman Stick are tuned in 5ths. A typical root-position triad played on the bass strings is voiced root/fifth/tenth (the tenth being the same note name as the third).