The “Local Control” would not affect the situation... that’s a MIDI thing. Think about that... “local” refers to the connection between the MX keyboard and the MX tone generator being in the same box. When On the keys address the synth engine as normal. When Off only the MIDI messages travel Out... the lack of sound is because you have disconnected the local control... now Midi messages that are sent Out must be echoed back to the MX THRU the software in order to trigger the synth tone generator.

Know this... the MX generates two audio streams... one analog that travels via the 1/4” Outputs, one digital that travels via the usb cable.
It is not completely clear to me how you have things connected. But if you are hearing two signals, that’s not unusal because there are indeed two pathways. There are two audio streams — your task is to ‘choose’ to monitor the one you desire.

You state you want to use the plugin effects in Mainstage... therefore you are interested in the digital signal path.. because the direct analog path would not/could not include any computer-based plugin effects unless you convert it to digital. The analog signal travels at speed of light to the next link in your analog audio chain.
This direct analog signal is what you want to mute.

The USB signal path travels via the USB “To Host” port into the computer or tablet device running your software. If the MX is connected analog to another audio interface, then it is this USB version of the MX audio that you want to follow and monitor. This is the signal path you wish to Monitor. This path is available, in the computer, to be processed by other plugins (make sense?). Follow the signal flow... as it returns to your Audio Interface before finally being converted to analog.

If you can identify which path is which, and what is your actual audio interface, and how you can select Monitoring option in your configuration... I don’t know what the significance of being setup as an “external instrument” has on your setup.

Sorry, I can’t do MainStage Support (I don’t know it well enough) but signal flow is signal flow. Doubling is normal, the solution is to monitor just one of the signal paths.

Hi Phil -
Thanks for your reply. It got me wondering if I'm thinking about this the wrong way. The more I listen, the more it seems to me that I'm hearing the MX passing through the channel strip of Mainstage. I thought that in muting the channel strip I would mute the audio completely but I think what I'm hearing is the 'dry' signal from the MX not passing through the channel strip but only through my audio box. Audio will be passed either through the channel strip or not. Since I don't hear any kind of Doppler effect I'm certain my wrong expectation of what was going to happen got in the way and everything is as it should be.
One other question though: I use a Motif XF at church and wanted to use it to control Mainstage. When I downloaded the drivers, the Motif didn't show up as multiple midi channels the way it had with the MX. After doing some research, I didn't check to see if midi on the Motif was assigned to USB. Should that solve my issue? Thanks.

Sorry, I don’t do MainStage support, but the MIDI I/O = USB should be your setting if connecting the Motif XF to a computer via USB. You would set it to FW if connected via FireWire, you should set it to MIDI if connected via 5-pin MIDI to an interface. But in all cases the Motif XF would be 16 channels. MIDI is MIDI no matter what type of cable you send it down.

The Motif XF uses a different Driver from the MX. The MX uses the “Yamaha Steinberg USB Driver” for both MIDI and Audio.

Make sure you have the correct driver for the type of MIDI connection you are using.
If connected to your computer via USB — Then make sure you have the “Yamaha USB-MIDI Driver” version 1.3.2-2 for Mac or 3.1.4 for Windows. This handles MIDI for the Motif XF. The XF does not do audio via USB

If connected to your computer via FW — Then make sure you have the “Yamaha Steinberg FW Driver” version 1.7.5 for Mac or 1.6.6 for Windows. This handles MIDI and Audio for the Motif XF via FireWire.

I installed the correct Motif XF drivers and set midi:USB. I'm seeing only channel 1 for the XF but no multiple channels. The setup in Mainstage is the same for the MX: I created a template to access multiple midi channels and assigned midi to YamahaMX port 1. This gave me multiple midi channels for the MX. I did the same with the Motif but only channel 1 is available. Could there be a driver conflict with the MX or something I'm missing? Thanks.

Something your missing, most likely.

The MX is always in a multi-timbral, Multi-MIDI Channel mode.
The Motif XF is only in a multi-timbral, Multi-MIDI Channel mode when you are in SONG MIXING or PATTERN MIXING modes. Both VOICE mode and PERFORMANCE mode are Single channel modes!

Thanks. Yes, I'm aware. I used my Motif ES as a controller for Mainstage in song/sequencer mode for years to take advantage of the tracks also being midi channels. That's why I bought the MX. Regardless of the mode you're in though the Motif XF would still show up as having multiple midi channels in Mainstage. So no, that's not the part I'm missing. I was wondering if there could be a driver conflict with the MX. If not, I'll try another clean install of the Motif driver and start over with Mainstage

Sorry, sounds like your issue is with MainStage.