There is no facility to route just low end signals to a subWoofer. That is, the Montage does not have a built-in crossover. What is normally done with studio monitor systems that feature a sub-woofer is described below...

Not sure what your subWoofer manufacturer recommends but say you were using the Yamaha HS-series with SubWoofer (HS8S), you would plug the LEFT and RIGHT MAIN outputs of the Montage into the Yamaha SubWoofer inputs (it has both XLR and TRS 1/4" inputs), it has a built-in crossover and will send (OUTPUT) the mid and high frequencies to the full range cabinets... which come in HS5, HS7 and HS8

For more info:
HS-Series Monitors

I think he's referring to the Montage subwoofer feature. I saw that somewhere as an optional setting. Actually, I might be thinking of my TF1. Trying to learn both simultaneously.

Thanks guys - I understand how frequency crossovers work etc. Perhaps more succinctly, my Q is -- how do I get the Montage to send ANY particular line level output signal to its "assignable output" jacks in the rear? They appear to be dead in the factory settings, and I can't find where to configure them in the Utilities menus....

Any PART, including the A/D Input Part can routed to any of the OUTPUTS.
These include the default "Main L&R", the "Assign L&R", "USB Stereo" (15 pairs), "Assign L", "Assign R", "USB Mono (30 individual).

Naturally, all Parts default to the "Main L&R" outputs. They are not "dead", you've yet to assign anything to them.

The "Main L&R" are considered the System Outputs. They include the System Reverb, the System Variation, the Master Effects, and the Master EQ.
All of the others are considered "direct" (isolated) outputs and include only the assigned Part's 3-Band, 2-Band and dual Insertion Effects.

The "Assign L&R" are analog, the others are digital outputs.
These assignable outputs do NOT include any shared (System or Master) effects, only the assigned dual insertion effects.

The Output assignment is always found as a PART parameter, and is made on a per Part basis.
It can be found in several places:

Select a Performance
Press [EDIT]
Press a PART SELECT button
In the lower left of the screen "Common" should be blue.
Touch "Part Settings" > "General" > PART OUTPUT is found here.

It is also found on the "Effects" > "Routing" screen, which always gives you an overall view of the signal flow for this Part, left-to-right.