1723 Bookshelf S vs 1723 Monitor S

What are the differences between our 1723 Bookshelf S and 1723 Monitor S?

Our 1723 Bookshelf S is a sealed 2-way design. 
With this driver arrangement and the help of our waveguide design, the radiation pattern is much wider than on our MTM style (Mid/Tweeter/Mid) Monitor S.
This means a bigger soundstage on both the horizontal and especially the vertical axis, with a very enveloping sound, and the speaker virtually disappearing into the room.
A very big advantage in small rooms and close listening distances. The sealed design also contributes to the small room benefits. It lets you place the speaker closer to the walls without the sound getting too boomy or muddy. For a full-range sound, we recommend a subwoofer though.

The 1723 Monitor S is also a 2-way but in a vented MTM configuration.
The MTM or D'Appolito design helps focus the sound and reduces reflections from the floor and ceiling. This creates a much more direct sound with fewer room interactions and is especially beneficial with greater listening distances and in bigger rooms. 
In addition to that, the two Mid/Bass drivers in its vented cabinet minimize distortions, raise the SPL capabilities and lower the bass extension quite a bit compared to the Bookshelf S. 
It is a great full-range sounding all-rounder, even without a subwoofer.

In conclusion, both speakers have their pros and cons and it will depend on the individual use case and personal preference of which speaker will be right for you.