Thanks for the question.

All 128 Performances on the MX are in the USER Performance Bank... and therefore can be replaced with your selection of Performances.

When you SAVE (create a File) to a USB stick, the MX will SAVE ALL “stored” Performance data in an MX ALL data file .X5A
In MX-speak, the words SAVE and STORE have different meanings:
SAVE is the act of writing a File to a USB memory stick.
STORE is the act of writing a program to the MX’s internal User memory.

Only STORED Performances are SAVED.

Once you have an MX All data file .X5A, you can either LOAD ALL data, or you can select to load a single Performance from within that ALL data file. This is how you can assemble those that you want from any .X5A File.
In other words, you have LOAD OPTIONS that allow to replace all Performances or you can select any single Performance and then you can target any one location to overwrite.

See Page 62 of your Reference Manual states... (Read “Load” pages 61-62 for full details).
“A specified Performance in an “All” file that is saved to USB flash memory can be individually selected and loaded to the instrument. When selecting this file and then pressing the [ENTER] button, the “Src Performance” display (for selecting the Performance to be loaded) and the “Dst Performance” display (for selecting the Performance destination) are called up. After making settings in each display as desired, press [ENTER].
NOTE
The loaded data is Performance, and does not include User Voices.”

“Src Performance“ = Source Performance
“Dst Performance” = Destination Performance

Load the larger amount of data as an ALL File, then load, one-by-one, the 3-5 Performances from the B.O.M.B. Bank — You can select which of the 128 you wish to overwrite with each one as you load it.

Hope that helps.

Thanks very much -- the distinction between SAVE/STORE was what I missed before. I now have downloaded the relevant pages from the Ref manual.

I went for the MX because of its straightfoward setup and I'm not disappointed....but I do wish it had an entire 128-slot user memory area that was fully accessible alongside the factory performances.

thanks again
TM