Well, here's the official YouTuber from Yamaha 😉 and he's pronouncing "Moe DX":

As the old joke goes, “Call me anything but late for dinner!”

In different parts of the world the name maybe said differently. I’ll give you the history... (then you decide) in 2001 Yamaha introduced the flagship Motif6/7/8 (Integrated Sampling Sequencing) synth workstations, and in 2003 replaced it with the much improved Motif ES6/7/8/. Shortly thereafter, in 2005 a cost down version of Motif/Motif ES (sans the sampling, mLAN and PLG Board expandability) was introduced, called the “MO” — as the MO6 and MO8 (pronounced by almost everyone as “moe-six”, and “moe-eight” ). The MO had the identical Wave ROM of the flagship Motif ES.

In 2007, the flagship Motif ES6/7/8 was replaced by the Motif XS6/7/8, the architecture went to 8 oscillators, DIMMs, mLAN/FW, reinvented Arpeggio, etc., etc.
And then in 2010 it was replaced by the FLASH Board improved Motif XF. Pretty much the same engine as XS with double the Wave ROM and the more reliable on board sample storage. The “F” was for Flash.

Shortly thereafter, in 2011, came the cost down MO-series versions of the Motif X-series (exact same Wave ROM as the Motif XS) featuring many of the flagship’s features, the MOX. MOX6 and MOX8. It was at this point you had some calling it the “mocks” and not the “moe-ex”. We called it “moe-ex”

Then in 2013, Flash Board expansion was added to the cost down models in the MOXF6 and MOXF8. If you called MOX6 and MOX8 the “mocks-six” and “mocks-eight”, I imagine you continued with that in the Flash equipped models... and just added the “F” for Flash. As in “mocks-eff-six”

We at Yamaha USA called them the MOXF (moe-ex-eff) and we call the MODX (as in moe-Dee-ex)... the DX7 reference should be clear to those old enough to recall the groundbreaking FM digital synth. The MODX6/7/8 are, of course, the cost down versions of the MONTAGE6/7/8.

If you know the relationship between the Motif-series and the MONTAGE-series, it’s fairly easy to see the relationship between the MO-series and the MODX-series. “Mod” as in short for “modern” never comes into play in any of these keyboards... so go with Motif > morphing into the shortened nickname-like “Mo”, and given the Motif XF and its relationship via the “Flash expansion” to the MOXF. I think it’s clear, but there is no wrong in this...

Call them whatever you are comfortable calling them - it all depends on where in the world you live... but whatever you do, call them!