Assignable outputs will not have master or system effects applied. These effects may be boosting the signal for Main L&R. Many effects have a programmable gain stage associated with them.

I don't know if the level presents a problem or just a curiosity.

Also, see the first picture in BM's post:

https://www.yamahasynth.com/ask-a-question/how-do-i-record-mono-vocals-guitars-thru-montage#reply-95985

ADIN.PNG

And notice the parameters "Output: Main L&R (+0dB) Assign L&R (+0dB) USB Main (+0dB) USB 1-30 (+0dB)"

You can individually boost or trim any of these outputs to get different relative baseline levels.

In order for sound to come out Assign L&R - you have to route it there. I imagine you are handling this fine.

Google Translate:

Hello. maybe I have a problem in the montage.
I bought the keyboard I have a month ago, and I would like to know if I should send it to the service.
The main outputs send much less signal than the assignable ones.
that's normal?
I already ruled out that it is a cable problem. the console channels that receive the signal have the same parameters.
Greetings.

Without writing out your methodology (exactly how and exactly what test signals you are running) it is very difficult to verify if what you surmise is a problem or just an unrealistic expectation.

When a Part’s signal is routed to “Main L/R” it travels through the (2) System Effect blocks (Reverb and Variation), the Master Effect, and the Master EQ prior to going to the analog Output. Sending that same signal to “USB1/2” should/ could initially sound different. All of the missing blocks can influence the Output level and the sound. We can completely eliminate their influence as follows:

Try this experiment:
Start with an initialized Performance.
From HOME:
Press [CATEGORY SEARCH]
Set the “Bank/Favorite” = All
Set the “Attribute” = All
Tap the “Init” option
Select “Init Normal FM-X” Performance. The Init Performance has only Rev Send 12, Variation Send is 0, there is no Master Effect, there is no Master EQ. If we defeat that Rev Send, signal sent to any of the buses will sound exactly equal in volume.

This will send a Sine Wave to the Outputs.
Press [HOME]
Press [EDIT]
Press [PART SELECT 1]
Touch “Part Settings” > “General” > “Part Output”
Initially, the Part is set to “MainL/R”
On this same screen line, set the default RevSend = 0 (default is 12)

If you now change the “Part Output” to any of the USB stereo pairs, the Output level will be exactly the same as the level going to “MainL/R”. This will prove the signal routed either directly to the Main L/R analog outputs, or assigned to a USB bus is equal

Selecting “AsgnL/R” — You will hear nothing unless you connect speakers to the “AsgnL/R” on the back panel. Any Parts assigned to the “AsgnL/R” are also removed from the Reverb Effect, the Variation Effect, the Master Effect and the Master EQ.
The ability to Monitor the “USB Main” and USB1-30” Outputs is provided as a convenience.... (so you can hear yourself when recording via the digital output). You can never listen to the digital signal.

Extra Credit: Why the “USB Main” and “USB1-30” Gain parameters seemingly have no effect on what you hear
You will find the monitored audio Output remains unchanged if/when you change the [UTILITY] > “Settings” > “Audio I/O” > USB Output Gains... (as mentioned by Jason)
The reason changing these seemingly has no effect on what you hear is because in order to hear these effect the sound of signal routed via the USB buses, would mean you’ll need to monitor post (after) the signal arrives in your DAW and is *returned* to the MONTAGE — you’d need to be monitoring the Audio Return (signal routed through the DAW and returning to the MONTAGE acting as audio interface) — the level change is applied to digital audio before it exits the MONTAGE via USB — it is not applied to the analog version of signal going directly to the Analog Outputs.

Extra Extra Credit:
If you are ever looking to boost or cut (for that matter) an individual Part’s Output, the MONTAGE provides a convenient -/+12.0dB adjustment per each Part. This is located before the signal reaches any of the Output assignments, and is located in the “EQ” block just after the Part’s Insertion Effect blocks. This will be an actual boost that can be monitored in the direct signal and is sent to the USB bus Output, if assigned.

While in Part Edit, navigate to the “Effect” > “EQ” > select the “2-band EQ” > find the “Output Level”
Boosting or cutting the Part’s Output Level here will affect both the Direct (analog) Out, and the Assignable (analog) Output, and the Assignable USB (digital) Outputs (both USB Main and USB1-30).

Troubleshooting Volume issues
A common user error concerning MONTAGE/MODX Volume Output, is inadvertent changes to the instrument’s Main Volume parameter.
Press [UTILITY]
Touch “Settings” > “Sound” > make sure the instrument’s TONE GENERATOR “Volume” = 127

This volume can be changed by playing a MIDI File that includes a Universal MIDI Volume message — used in Standard MIDI Files to create a “fade out” at the end of a Song (sequence). Unless you send a RESET command, you will want to manually return this to 127

Gracias por contestar tan rápido a Uds.
Ya hice las pruebas sugeridas y continúa la diferencia de volumen.
Lo que percibo claramente es que las salidas principales tienen menor volumen que las asignables, por lo que supongo que los efectos no están causando la diferencia. En ese caso la diferencia sería a favor de las salidas principales.

quisiera agregar algo. tengo un motif xf a la par del montage. hice las mismas pruebas en el motif, y el nivel de salida de las asignables es exactamente igual a las principales.

Google Translate:

Thank you for answering so quickly to you.
I already did the suggested tests and the volume difference continues.
What I clearly perceive is that the main outputs are lower in volume than the assignable ones, so I assume that the effects are not making a difference. In that case the difference would be in favor of the main outputs.

I would like to add something. I have a motif xf next to the montage. I did the same tests on the motif, and the output level of the assignable ones is exactly the same as the main ones.

I repeat... please check Main Volume: because this would affect the Main L/R Output, but not the Asgn L/R Output.

Troubleshooting Volume issues
A common user error concerning MONTAGE/MODX Volume Output, is inadvertent changes to the instrument’s Main Volume parameter.
Press [UTILITY]
Touch “Settings” > “Sound” > make sure the instrument’s TONE GENERATOR “Volume” = 127

This volume can be changed by playing a MIDI File that includes a Universal MIDI Volume message — used in Standard MIDI Files to create a “fade out” at the end of a Song (sequence). Unless you send a RESET command, you will want to manually return this to 127

Si. ya comprobé eso también.
Envié el teclado al service.
Muchas gracias por intentar ayudarme a resolver el problema. Cuando tenga novedades, les haré saber si yo estaba en un error o el teclado funcionaba mal.
Saludos.

Google Translate:

Yes. I already checked that too.
I sent the keyboard to service.
Thank you very much for trying to help me solve the problem. When I have news, I will let you know if I was in error or the keyboard was malfunctioning.
Greetings

Let us know.