What do you mean by sub bass? Are you referring to a sub harmonic of the instrument?
I will assume you are asking whether an instrument can produce a pure sine wave.
It would be very unlikely to get a pure sine wave from a musical instrument. Even a function generator will produce transient harmonics due to being "switched on". Without getting into the details of the math and physics, the specific harmonic spectrum of the instrument depends on (1) what overtones are supported by the physics of the instrument construction and (2) how the instrument is attacked, or how sound is excited in the instrument. Additionally, the harmonic spectrum can depend on time. It may be bright at first then decay, leaving mostly the fundamental (which would be as close as possible to a pure sine wave). The guitar and other plucked instruments have this property.
If you pluck a guitar string close to the center you excite more fundamental but you still do not have a pure sine wave. I would say that it is very unlikely that any instrument and player could produce this. At the very least there would be harmonics for a short time due to starting the sound.
As a side note, the human ear is non-linear and creates aural harmonics. So, even if you use a function generator as an input the signal to the brain will be "corrupted" with harmonics.
If you are referring to an instrument producing a true sub-harmonic. I is possible but most so called examples of a sub-harmonic are false. They are the result of changing boundary conditions in the attack and people completely misunderstand the physics of what's going on.