Well, Christopher Page has undoubtedly researched this matter in great detail, and doesn't arrive at his conclusions merely at a whim.

Well, is there any more historical evidence you could present for that conclusion than might be presented for it having been sung by choirs? Vocal forces in medieval music isn't one of my specialist areas, but I do know that there more than a few scholars who have investigated it as thoroughly as seems possible.

All those arguments are indeed identified by Taruskin, and in some detail (which is not to say I agree with all his conclusions). And by others since him, as well.


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Sydney Grew the Elder has a few words to say about the authenticity of ancient music. We especially approve of his emphasis upon the ideal the universal and the impersonal:

The old music is the final expression of the first, or Catholic, manifestation of the Christian religion. Growing out of the Middle Ages, the music is a thing of rapture - the fruit of a passionate conviction, of the long-sustained contemplation of truth as truth was then perceived, and of the ecstatic appreciation of the significance of that conviction. It glows with love, and throbs with spiritual life. In it are the variety, colour, freedom, and stability of Gothic architecture. It is the parallel of the English of the 1611 version of the Bible.

These facts are not yet altogether recognised. Indeed, so little is the character understood of even the period itself from which the music flowed, that some of its recoverers still speak of "the asceticism of the Middle Ages," which had to be destroyed before "the natural instinct of man found expression in a paradise of eternal happiness." I think you may believe that the part of mediævalism which gave birth to music was not the dry and arid part, but the part which from the beginning was already in that paradise of eternal happiness. While the character of the cause of a thing is misunderstood, the thing itself must remain misunderstood still more.

We say the old music is mystical. But we do not mean that it is indirect. In one sense, the word mystical means allegorical: when we have a spiritual subject, for example, that cannot be defined by words or reduced to intellectual ideas, we express it by allegory or metaphor. Thus the conception of the love and unity which subsist between the Soul and its Saviour, also the emotion attending upon a believer's contemplation of this love and unity, are expressed allegorically by naming the Saviour "the Bridegroom." Language here is used to a mystical end. Poetry is very largely mystical, according to this explanation; but music is not.

Nor do we mean that the old music is esoteric, reserved for the understanding of the specially informed, or codified for the information of the secretly instructed. If it were so, it would not be art. The earliest mediæval notation, indeed, is very secret, and even to well-educated modern musicians the notation is as obscure as Assyrian hieroglyphics. The music, on the contrary, is open, and all-revealing to those of us who by nature are in sympathy with it.

We mean when calling the old music mystical, that it is first ideal, secondly contemplative, and thirdly elemental or universal. By ideal I mean pure; by contemplative I mean non-realistic; and by universal I mean impersonal. The art expresses a complete and un-self-conscious surrender of the artist to his subject, and therefore it becomes the subject itself. These descriptions are not easy; you will understand them as you learn to analyse your musical experiences. For the moment, it should be sufficient to add in further explanation merely that a smile or an instinctive gesture is in nature as music of this kind is in art.

I've often been vexed by this question, particularly since I don't find Christopher Page's "solution" particularly convincing. His recorded performances are certainly very accomplished, but the scientist in me humbly suggests that all-vocal performance of that repertoire involves more assumptions than vocal/instrumental performance. A few matters which occur to me are:

(a) Page writes of the work he put into devising a suitable constant vowel sound for non-texted voices. Since there are extant musical treatises from the middle ages, one might expect such an important issue to have been mentioned somewhere.
(b) Intonation - can it really be true that singers of the time would have been expected to keep that music in tune without the help of instruments? Somehow I doubt it, especially since intonation was something that theoreticians wrote quite a lot about so it was probably considered important to get it right.
(c) Most contratenor parts are clearly different in shape from the others, one might say less "vocal" in shape.

None of the evidence in either direction is conclusive, it seems to me, and it's still the case that all-vocal performance of mediaeval secular music is mostly a British phenomenon, whatever that might imply.

I certainly don't know of any theorist of the period who mentions humming or vocalising the non-texted parts.  I wouldn't go so strongly down the path of needing instruments to "stay in tune"... there was certainly a corpus of ecclesiastical music in which instruments probably didn't feature, so if they could stay in tune there, by extension they should have been able to do so in secular music too?   Another related issue is range and "top down aural approach".  Almost all ensemble singers these days will "tune in to the bass line" and ensure they are in tune with that.  But medieval chanson often doesn't work like that, and is frequently for three equally-pitched lines, with no line that has the structural "top-down" function of a "bass line" (which is a phenomenon of a different era).  And in fact, the number of such parts which exploit the bottom fifth of the bass clef (ie below "viola-c") is relatively few...  yet voices could always have "gone there"?  You could hypothesise that this was to keep the untexted parts within the range of stringed instruments?

Of course, we come from an age in which notated scoring is sacrosanct, and woe to the trumpeter who plays a C-trumpet part transposed on his Bb instrument....   but the Middle Ages had no such protocol, and was driven by a healthy pragmatism...  it's very possible that different line-ups were used on different occasions, even by the same performers (who were all expected to be multi-instrumentalists anyhow, and be singers too). We know for certain that vocal music was performed instrumentally - there's the famous case of Kalenda Maya, upon which the two fiddlers improvised to the crowd's approval.

Binkley, amongst others, made the early conclusion that those angular contratenor parts might be instrumental, as they are so unlike conventional vocal music.

It was just a thought that occurred to me, and it seems to be a leaky one I admit. Far more persuasive is the idea that those people must have been doing something with their instruments (passing for a moment over Hieronymus Bosch's ideas on the subject).