The LFE is merely part of the receiver’s overall controls for adjusting the levels between the various speakers and the subs. A lot of it has to do with the efficiency of the speakers one might have.
For instance, if someone is using speakers for their system from different brands and/or models, some will probably play louder at a given volume setting on the receiver than others. You’ve probably noticed that the Z9 has a function in the menu for adjusting the levels of the different speakers. That’s what it is there for, to make sure all the speakers, especially if they are mismatched, are all able to play the same volume.
The LFE works along the same lines, simply allowing an outboard powered sub to be level-matched properly with the other speakers. Similar to the speakers, the inputs of some subs can be more “sensitive” than others (probably not the most accurate description from a technical standpoint, but good enough to make the point here). In other words, some subs may require a higher LFE signal than others to achieve the same volume level.
More importantly, the LFE output can be very helpful in some real-world situations. For instance, if you have extremely efficient speakers (some Klipsch models for instance), you can get ear-bleeding levels in the room with a relatively low volume setting on the receiver. In that situation you would have a problem getting a hot enough signal to the subwoofer. IOW, even with the sub’s own gain control turned all the way up, the LFE signal from the receiver might not be high enough to drive the sub to its maximum output, or maybe not even enough to get the sub loud enough to blend with the main speakers. So, the LFE output in the receiver can be increased to compensate for this situation, enabling a proper blend between the mains and sub.
Conversely, if you have extremely inefficient speakers you’d require a relatively high volume setting on the receiver most of the time. In this case you could be sending too hot a signal to the subwoofer, possibly overloading its input (causing distortion), or maybe rendering the sub’s gain knob to “hair trigger” operation, going from nothing to full-blast with only 1/4" of the knob’s travel.
Make sense?
Regards,
Wayne A. Pflughaupt