A button to toggle the player (mplayer vs. mpv, maybe more, e.g., ffplay, but only if that remotely makes sense) would be nice. Sometimes I end up in a silly state: mpv can play, test mplayer, FAIL, back to mpv, FAIL. Just delete all saved playback INI-files got me back to mpv can play. Of course this affects only exotic formats (e.g., SMjpeg) and codecs (e.g., Dirac).
JFTR, the portable SMplayer 16.6 is shipped with mplayer 37871, the next 37872 also works. It's also shipped with a vintage 2015 mpv 0.11, on Windows 7 x64 0.17 also works. Maybe SMplayer for Windows uses an older mpv for XP (wild guess).
Updating a portable SMplayer is a major operation for me, it consists of up to 5 downloads (SMplayer, freshest mpv, freshest mpv, freshest SMtube for SMplayer, freshest youtube-dl for mpv), and so far I have not figured out why I should use SMtube + youtube-dl at all, I have a nice one-line Savedeo favlet/bookmarklet to handle tricky cases like "mux best audio with smallest dummy-still-image-like video". IOW, the SMtube docu sucks.
javascript:void(window.open('http://savedeo.com/download?url='+location))
If after changing from mpv to mplayer and mpv again, a video fails to play I suggest to uncheck the option "remember settings for all files" in preferences -> general.
The Windows packages include the old mpv 0.11 because recent versions have problems with online streams, for example youtube live streams.
SMTube doesn't need youtube-dl. Youtube-dl is used by mpv for playing videos from streaming sites like dailymotion, etc.