While the MX is not officially GM or XG compatible, it does contain a pseudo-GM Bank that is constructed to respond to standard GM Program Change numbers. It will be able to select an appropriate sound for any of the GM/XG setups.

An "Illegal File" error message can be caused by many other things... it can depend on how the file was prepared, what data was included in it... and so on. It would be impossible to pin point the exact cause without knowledge of the file's origin. Make sure the extension is exactly .MID

If you are using a DAW software to create your own MIDI files you can have the file fully automate playback... selecting sounds, adjusting levels, effect sends, pan position, etc., etc., etc.

We can post you an example, if you'd like...

Hi guys, I've found an anomaly when playing back GM midi files on my keyboard. On some files the sysex setup data resets channel 10 (drums) to a standard GM instrument. I looked deeper and it's only on midi files with Roland GS commands embedded in it. It's easy to fix, you can either increment the bank to GD (GM Drums) after it starts playing or you can fix the file permanently using a sequencer by removing the Roland commands.
Just an observation that might help someone else in the future.

True, the Yamaha Synthesizer keyboards do not necessarily respond to the Roland GS format (but more likely it is the failure to include both the MSB and the LSB values for Bank Select or the lack of GM REST command). If the file is standard GM or even Yamaha’s own extended GM spec, XG, the MX will do the right thing, but MSB *and* LSB must be usedfor proper recall and GM RESET command is necessary at the very start of the file. For more, in general, on what constitutes a proper GM File please see the following... (although it uses the MOXF as the example) the information concerning file data is on point:

https://yamahasynth.com/component/k2/working-with-and-creating-proper-gm-files

The Yamaha Synthesizers are not fully GM/XG compatible, they will call up the the basic instrument that corresponds to the 128 Normal bank of GM programs, and it does feature one GM Kit. The *focus* in the Yamaha synth products is toward substituting your own selection of sounds for the basic GM listing. There are many (strict) requirements that must be adhered to in order to get that official GM/XG logo, we chose not to restrict the number of Oscillators, or number of layers, etc, to meet the requirements, but did substitute sounds that are not from some separate card or chip, we simply have created a program list using full ROM sounds that substitute the appropriate instrument for the GM/ XG principal Program numbers.

I recently downloaded tons of midi files of various famous songs, movie soundtracks, tv shows.
The files that did play back correctly from the MX49 sounded really good.
Even general midi sounds great on the mighty MX 🙂