I don't think the 25 strings are directly related to these myths. The se (very similar to a modern qin/guqin, with strings over a baseboard) was, mythically, originally a 50-stringed instrument, but the sound of its playing was so sad and moving that Emperor Di ("Emperor Emperor") could not bear it, snapped the instrument in half, and from that time on, it had 25 strings only.
So the "25 strings" of our poem simply means "a se." The origin myth of the se was well-known enough, though, that the reference would make listeners think of mournful, funereal music.
You're right that this is important background. I'll add it into the notes and think about whether more information can easily be incorporated into the body of the poem.
What's the significance of 25 strings to (presumably) the latter myth?
I don't think the 25 strings are directly related to these myths. The se (very similar to a modern qin/guqin, with strings over a baseboard) was, mythically, originally a 50-stringed instrument, but the sound of its playing was so sad and moving that Emperor Di ("Emperor Emperor") could not bear it, snapped the instrument in half, and from that time on, it had 25 strings only.
So the "25 strings" of our poem simply means "a se." The origin myth of the se was well-known enough, though, that the reference would make listeners think of mournful, funereal music.
You're right that this is important background. I'll add it into the notes and think about whether more information can easily be incorporated into the body of the poem.