When you have a correct treated room, subwoofer are not realy needed.
But Subwoofer are the way to go, specially in untreated / not perfect rooms, like living rooms,
to get an smooth freq response acrosse the freq spectrum.
Just google Single / Double Bass Array / Pseudo DBA
and how they works, I would never want to hear without Subs.
Yes, its alot of work with the DSP and measuring but the results are
(for me) way better than just listening with my pairs of KH150ˋs.
I sold my KH750 recently to an guy who has KH420s in a fully treated room,
but he wants just the last few % to get an better lowend.