If the user wants to add a derived, low-passed subwoofer to their system, the Slate Control stereo version (standard model) can do this easily and at-will on any speaker pair. The LFE output is taken from a sum of the control room out. This is called a "derived sub." There is a low-pass filter, a phase flip, and level control.
The surround unit adds the option of a discrete LFE input from your DAW or console, and add the ability to switch between discrete and derived subs.
Sometimes the terms "LFE" and "SUBWOOFER" are used interchangeably but they are different in a few key ways. For many users, LFE implies a discrete send and SUBWOOFER implies a derived send. We have chosen to use the "LFE" nomenclature on the Slate Control to describe both discrete and derived signals, as both exist on the surround version. The stereo version has a derived subwoofer only and does not accept a discrete LFE input.
The mains are not high-passed by the LFE circuit in any way. This is important so that users can retain the full range of their monitors and add the low-passed sub, in the manner that many studios and listeners prefer. This also allows the sub to be calibrated with acceptable accuracy across several speaker pairs, without changing the sound of the speakers and ensuring that the fundamental tone of the monitor doesn't change when adding the sub temporarily. When a user wants to create an extended-frequency response full-range pair that is perfectly calibrated, we recommend building that system _after_ the Slate Control and treating it as a single pair. I will be happy to provide diagrams.
The LFE can be added to any of the three speaker outs at-will by toggling the "LFE" switch above the speaker selector.