It was simple for me...bass mgmt 101.
Speakers (even 'full range) do not necessarily produce full range
Effectively. Many speakers struggle to produce full range bass. They might
Not be able to produce the same db as the rest of the frequency range
Presented by the speaker. Also, distortion may start to rear it ugly head.
The positioning of the speakers in the room is dictated by the listeners
Seating place. This is not always the best place for 'ideal' reproduction
Of lower octave frequencies. The ability to place a subwoofer (hopefully)
In a different place in the room can present a much more linear frequency
Response. Proper use of bass mgmt can also 'clean up' the midrange
As the driver may not have to 'struggle' to produce bass heavy material
Along with mid-range frequencies. The subwoofer can do this much more
Effectively. (Especially true for two-way bookshelf speakers)
I also feel that many of today's towers can't reproduce the bass frequencies
That speaker of the 70's could. And if they do they are big bucks!
I feel that trying to blend a subwoofer into a system with no bass mgmt
Is a compromise...mostly because it possibly defeats all the advantages
Listed above. And then there's "double bass" when U have speakers and subs
Trying to play the same bass frequencies. (Because you have no cut off!)
My 2-channel music set up was built around my choice of the
Outlaw Audio 2150 receiver....which has analog bass mgmt.