Subwoofers will not add clarity to a system. They will add impact in the form of notes you can feel as well as sound stage volume.I’m not sure of the logic behind this statement, but it is at least subtly wrong. In a small two-way system, the woofer covers both (sorta) bass and mid-range. The large excisions of the bass modulate the mid-range, since the mid-range is superimposed on a vary-ing, larger, bass excursion. This HAS to muddy and distort the mid-range. And in my listening, it does (or did, its been a while)
Beyond that, yes, it will add weight, impact and richen the timbre.
Now, all this depend son proper setup, and you need to remember that the subwoofer must be phase aligned with BOTH speakers, and that phase alignment is perfect only for one listening position. Do the math. Its worth spending some time to get the position right, and if the crossover may be varied, to find out which one integrates best with your main speakers.
I really like sub-woofer / satellite systems if all the above are observed.
Some of my fav’s from the way-back machine were Spica TC-50is and Sequerra Met-7s with a decent woofer such as an Audio Pro (about $1000, 30 years ago).
G