Jonathan knows better than I do, probably, but I would guess one doesn't accompany the Marian antiphon in its simple tone because --- well --- it's the simple tone. More elaborate ceremony requires more elaborate instrumentation, I would think.
I do not mean to enter into this by offering argumentation about the pluses and minuses, the correctness or incorrectness of 'accompanied' chant. I will, though, take issue with your implication that 'accompanied chant' represents greater solemnity than chant which is unaccompanied. Such an argument is outrageously presumptuous and without foundation - (or else humourous in the extreme).
As for Matty's question, Jonathan would seem to have a plausible answer. Perhaps one of our experienced scholars of the NOH could supply a definitive answer.
I don't mean more solemnity, but more elaborate-ness. I resist the idea that Propers ought to be accompanied and that Ordinaries must be accompanied. ( I also find unfounded the concept that the assembled lay faithful must sing everything and that the music should be chosen to elicit such participation ). Rather, I'm expanding on the idea that we bring out trumpets and such for specially festive occasions. Festive and Solemn aren't interchangeable terms, so I'm sorry for the confusion.
By the way, the simple tones not being part of the Vatican Edition means that the "solemn" tones were in fact intended to be used every day. Originally you will see that they had no designation, they were just the Marian antiphons and that was that. Because the simple tones existed as such, later the designation "solemn" came to be applied to the more elaborate tones.
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