Hi,
I think it was possible to Sideload MMA APK on it, but not sure it is still Valid

OK, thanks. I am contacting several sellers to see if they know. May contact manufacturer in Korea, too.

That would depend on the device having any limitations regarding folder/filenames. The manufacturer should know this. MediaMonkey can change folder/filenames on Sync to something that works for the device.

It not a file-type issue. I use FLAC files and the DAP handles that type. Its a matter of how the files are organized (I hope I'm describing the right thing.) To add music to the DAP, I copy files to the drive. I have a number of albums that I bought digitally, so I can copy them and the DAP will read them. My question is...will the music folder created by MM when I rip CDs be readable by the DAP? Are there conventions that everyone uses to ensure compatibility among various items? I don't want to buy a $3K DAP if I can't copy my 700GBs of MM music to it and use the music with all the tag data correctly displayed.

You'll need to check what file type(s) you have in your MediaMonkey Library and which file types the device supports. If you have incompatible files MediaMonkey can Auto-Convert on Sync to a format supported by the device.

I'm thinking of ditching my iPod and getting an Astell&Kern Digital Audio Player. I add music to the DAP by simply copying in files like it is an external drive. Is the MM "Mymusic" file where are my ripped music is stored compatible with the Astell&Kern DAP? Will the player recognize the files and display them properly? Sellers of the DAP do not know?