Send request to Yamaha after another gig and another sound man telling me
The board has crappy output signal.

Yamaha replied:

RICH,
Correct. The output is set a bit lower than our other products. At max the tested output is sending +/- 1.4 dbu. Unfortunately this can’t be changed. You’ve done about as much as you can do, but the unit wasn’t designed to send a stronger signal.
Thank you,
Isaac
Application Tech / Yamaha

Programming the audio output of the MX has been covered here already. (Perhaps this is a good subject for an upcoming tutorial) But the output of the MX is well within useable range (plugging into any amp or mixer) - the unit is set for use with multiple Parts playing back simultaneously. The levels of the individual Voices that get assigned to PARTS is programmable - you access this with the VOICE EDITOR. “Voice Volume” is different from “Part Volume”.

If you have individual Voices that you want to program for use in a “live” situation, I highly recommend that you invest in the VOICE EDITOR (Melas: MX TOOLS). You can STORE up to 128 custom USER VOICES in the MX. If you find a Voice that is in your top 128 I recommend you open it in the Melas VOICE EDITOR - tweak it to your liking - if you’d like to pin the meters on your sound person’s board, you can turn it way up… VOICE Volume is available in the EDITOR. Then STORE that in the “U” (USER) bank of your MX. Use the tweaked versions in your custom PERFORMANCES.

However, I find it hard to believe that your sound person cannot or does not have the gain to adjust for the MX (very hard to believe). While many of the Voices are conservatively set as to volume, it is not anything that major - the MX becomes a very, very programmable synth (like its bigger brothers MOX and Motif) when you access it via the EDITOR. Again, (the Melas “MX TOOLS”: highly recommended).

One of the things about the MX that is so very cool, is you really sacrifice very little. Sure you cannot edit everything you can on the MOX or Motif XF from the MX screen… But we anticipate that 97% of MX users will connect to a computer at some point. With the addition of the computer EDITORS you will gain access to parameters that you otherwise would not be able to access from just the front panel.

And just like the projects you record in your favorite DAW can go along with you to the gig as .MID or .WAV files (which can stream in real time via the USB slot), you connect to the computer, program powerful VOICES, and Performances that include all 16 MIDI channels if you wish… these new programs that you create with the computer can be stored internally in the 128 USER VOICE and 128 PERFORMANCE locations. Take them to the gig, leave the computer at home.

If you connect the MX to a computer (and we anticipate you will - the MX is its own audio interface and can act as a MIDI interface for an external device via USB) then take advantage of what’s under the hood… what’s really under the hood is accessible via the USB connection (the Editors).

Cudos to John Melas for his software.  they not only simplify but they are enjoyable to use.  indeed, there is much kore to the MX as I’m finding out. 
BTW, I too would find it very hardto believe if a sound person didn’t have the gain to adjust for the MX.  My guys fortunately had no trouble as I indicated.  Lets move onto something more positive about the MX!!!  Better yet, I’m going to practice......latera