The upgrade path to achieving high-fidelity sound reproduction in the home is sometimes convoluted, but the aim is high.
Simplicity may be a barrier to high-fidelity sound reproduction. The JBL Studio 590 loudspeakers have a rated frequency response of 35Hz – 40kHz (-6dB). Their -6dB point corresponds to the speaker being -3dB at 40Hz for a B4 vented-box alignment. That's not even enough to reproduce the lowest note on a bass guitar (41.20Hz), let alone a 5-string bass guitar (30.87Hz), and the lowest note on a piano (27.50Hz), with anywhere close to 100% fidelity as far as reproduction level goes.
Although the JBL Studio 590 loudspeakers do have considerable bass extension for a relatively compact enclosure size with their sensitivity, they are far from being able to reproduce a lot of low-frequency energy that some music, and many movies, will require on a regular basis.
Although the JBL 4367 may seem an attractive proposition, with regards to bass response it is only a little better: frequency response 30 Hz – 40 kHz (-6dB). Its design ethos is leaning more towards producing higher SPLs, rather than significantly more bass extension, notwithstanding its 15-inch woofer. As far as bass response goes, all other things being equal, the JBL L100 Classic, with a frequency response of 40Hz – 40kHz (-6dB) is the poorest performer of the bunch with regard to bass extension, even with its 12-inch woofer.
So, all in all, a subwoofer with a -3dB point of 20 Hz is still something that will provide better fidelity sound reproduction than simpler systems without a subwoofer. Of course, the tradeoffs between low-frequency extension and maximum output level need to be considered for the intended use case.