5 hrs on this and counting. I can record Montage through my signal chain by using the headphone outs. But there is no signal from the main outs.
I bet I'm gonna feel real stupid when I figure this out!! But thats just fine.

Strange because there's no way that I am aware of to target just the headphones vs. mains since the two are linked in terms of volume and routing. When your headphones are able to monitor and you hear sound - but Main L&R does not generate a sound - I would start to look at all equipment hooked up to the Main L&R - cables, mixers, preamps, downstream analog audio inputs, if going to a PC through a soundcard then all of those settings.

As a debug step - you can plug a powered speaker (monitor) directly into the main out L (then main out R) to see if you are indeed getting a signal straight out of the Montage or if the problem is further down the signal chain.

Also, not sure if you're using a USB out of the Montage. It's another possibility if configured as USB (audio output) that [UTILITY]; Touchscreen (TS): "Settings" -> "Audio I/O" ; TS: "Direct Monitor" - set to "ON" ... that the last step is set to OFF. From what I (think I) know about how Main L&R and headphones are linked - I would think that "Direct Monitor" would turn on and off both Main L&R and Headphones together. But I haven't used this feature enough to know if there are any exceptions to this.

My assumption is that the above scenario does not apply - and that the "rule" exists of Main L&R (technically: "OUTPUT (BALANCED) [L/MONO], [R]") output as "married" to the Headphone output.

Just continued to think about this from an electrical perspective: the headphone out jack is a stereo output with the tip/ring/sleeve connections very different from the balanced output of the (MONO) OUTPUT (BALANCED) [L/MONO] or (also MONO) OUTPUT (BALANCED) [R] jacks.

Balanced outputs carry a differential signal over two wires while your headphones use a single-ended (ground referenced) signal for both L & R over the same type of jack.

OUTPUT (BALANCED) [L/MONO], [R] (both the L/MONO and R side) have the following wiring:
Sleeve (side closest to the cable) = Ground
Ring (middle) = Cold (-) "Cold" is what is the negative side of the differential signal.
Tip (side with the point) = Hot (+) "Hot" is the positive side of the differential signal.

PHONES (headphones) have the following wiring:
Sleeve = Ground
Ring = Right channel (single ended, not differential)
Tip = Left channel (single ended, not differential)

So this may be more about matching the input to your soundcard if it does not accept TRS balanced and instead expects a stereo TRS such as headphones.

For physical connections: ... something like: http://www.redco.com/Redco-S8PM-XXBM.html for line or mic in 1-8. I'd have to spend a little more time to see what the signaling levels best match. The manual does show ULN-8 (page 97 of http://www.metric-halo.com/media/MobileIOUsersGuide.pdf ) has balanced inputs for both line and mic.

Also as an option are the two "DI" inputs on the front panel. You can use jumpers (on the ULN-8) to set the gain if the Montage out level is not sufficient. Montage also has an output level boost available for Main L&R which can also be used - either in conjunction or alone. I've had to boost my output for live use (using menu option) as the default setting is "low" relative to other sources.

The DI connectors 1&2 on the ULN-8 are shown as balanced TRS - so you can use a balanced TRS to TRS cable (male/male).

The DI connectors are "mixed" with Line input 1 and 2 (rear DB25 connector) - so it may be best to avoid connecting to Line 1 and Line 2 (rear) if using the front panel connections.

Thanks Jason! I did plug a speaker directly into the Montage outs with nothing. I also played V-drums into my ULN8 pres with no problem. SO, my single chain, cables and (MAC, Logic, and ULN) were working. Just not main outs of Metric Halo for some reason.

I took a break and went back and rebooted keys, ULN8 and Montage with the USB cable not connected to the Monty. VOILA!
Of course I have no midi with this setup but for now at least I can record the Montage and sax together for a session.

Any other thoughts on this issue certainly appreciated.

Now that you've mentioned using USB - take a look at that direct monitor setting. When USB is disconnected, it automatically switches to ON - which is what you want to get the OUTPUT jacks to send Main L&R audio.

5 hrs on this and counting. I can record Montage through my signal chain by using the headphone outs. But there is no signal from the main outs.
I bet I'm gonna feel real stupid when I figure this out!! But thats just fine.

Triple check your OUTPUT connections... I cannot tell you how many keyboard players mistake the Inputs for the Outputs. Looking over-the-top of the keyboard the jacks farthest to the left (low end of the keyboard) are the A/D Input (as you might guess, no audio comes Out of these) ... next is the Phones jack... then the Main L&R Output.... followed by the Assignable L&R Outputs.

You want the Main L&R Output!!! Labled: OUTPUT (Balanced) "L/Mono" and "R"

The Phones OUT and the main L&R are always the same. Always.

Thanks for confirming the "rule" - this is why I initially downplayed the USB monitoring as having anything to do with this. My understanding is based off of a hard-wired signal which "T"'s off from a buffer from the Main R&L to PHONES and OUTPUT L/R (possibly another buffer stage just to prevent a short on one from shorting the other output) - which would prevent any difference between the OUTPUTs and PHONES under all conditions. Since this is hardware equivalence that cannot be undone by any setting.

I could be wrong, though. No schematics in front of me.

"I did plug a speaker directly into the Montage outs with nothing."
So where is your amplifier?

And on your computer you will need to install a driver.
http://download.yamaha.com/search/detail/?site=nl.yamaha.com&language=nl&category_id1=16277&category_id2=16381&product_id=2166246&asset_file_language=EN&asset_id=60554

FYI: now that I have my board setup - testing Direct Monitor = OFF cuts off PHONES and OUTPUT [L/MONO], [R] simultaneously so there is no case where the outputs would not match the headphones. This is an underwhelming, "that's what I expected", kind of result that confirms the initial understanding of internal wiring which forces no difference between the outputs L/R and phones.

The only other alternative suggestion is make sure you're not panned all the way to the opposite side as your output connection. In which case stereo headphones would only sound out of one of the two sides.

I'm assuming within the 5-10+ hours you've put in by now that you always tried both L and R outputs - and probably tried plugging a speaker into the footswitch jack as well (which you knew was fruitless).