The option to download HIGH QUALITY VIDEOS (&fmt=6 in browser or -f6 in youtube-dl) needs to be enabled.

When youtube-dl-x (the default that accompanies mytube) is replaced with the generic youtube-dl (downloaded from the net) and the option -f6 is added to the settings under [preferences]
[more]
[youtube-dl] tab
i.e. (youtube-dl -f6 -o %d --%s) ,

pressing the play button downloads and plays high quality videos if available for that chosen video.

However, the "percent downloaded" counter is missing and playback does not start until the file is fully downloaded.

IS there a way to restore this functionality?

THe option to play a high quality version of a video whenever available would be the best feature.