Are you sure this is a good idea? It makes sense from one perspective, but what you really are doing is shoe-horning Mass VIII into the paradigm that has been established by all of those modern Mass settings that treat the memorial acclamation and the amen as being on par with the Kyrie, Gloria, Sanctus, and Agnus Dei: which they are not.

The Syrian custom of a chant addressed to the Lamb of God was introduced into the Roman Rite Mass by Pope Sergius I (687–701)

No doubt some objected vociferously.

Well if you are going to do this the Missa de Angelis is ideal, as it is a modern composition compared to the others!

Something like the Salvator mundi adaption I posted a while back seems to work better, by means of not cluttering up Mortem tuam with yet more arrangements. Having one distinct setting for each acclamation is the best we can do to stave off the nonsensicality (some might say idiocy) of having three options with no way of telegraphing to the congregation which is to be used.

Nathan - neither did I, until I started searching for Mass online while we were 'stay at home as much as possible'. Incidentally I notice Glenstal follow that with a rather poor rendering of the Our Father based on Rimsky-Korsakov. Unaccompanied they don't sound good, but if you look at their Saturday evening Office of the Resurrection they do much better with discreet organ to keep them in tune.
[Aside] anyone know of this Office of the Resurrection? Is it something they have adopted from the Orthodox?

People say

The Great ‘Amen’
St. Jerome once said that great “Amen” is a celestial thunderclap as heaven says “Amen” to this with us. This is the one and only sacrifice re-presented here on the altar and all altars around the world.

But is it true? Has anybody a reference, and I don't mean to an unsupported remark by Jungmann or any other members of the Liturgical Movement. The Exsultet speaks of the mighty voices of the people shaking the rafters, I like the idea that this refers to the assent of the people at the end of the Canon, but is it (or was it ever) rooted in our liturgy?

Historically, when composers would set the ordinary, they would set the Kyrie, Gloria, Credo, Sanctus-Benedictus, and Agnus Dei. Perhaps also the Deo gratias for the final dismissal. A random fact that did stick in my head from music history class, the first musically unified setting of the ordinary is the Messe de Nostre Dame of Guillaume de Machaut, which is from sometime in the 1300's.

The Benedictus was never a "response", because the consecration was silent as far as what was spoken by the priest. So there was nothing that was being responded to.

Side note: in De Musica Sacra (1958) there is already a provision that "the Sanctus and Benedictus, if chanted in Gregorian, must be sung without a break".

The Ordo Cantus Missae is the book describes how the chants of the 1908 Vatican Edition of the Graduale Romanum are to be arranged for use with the novus ordo Mass. The only chants which it provides are new items not present in the GR 1908. In the section "Cantus in Ordine Missae Occurrentes", the old Kyriale Romanum is incorporated by reference, and the selection of tones is thus left as it was in the GR 1908. On the other hand, the memorial acclamation and the amen, along with the priests parts that introduce them, are set forth in full, because they are novelties. For both of these, only one melody is given for the response. If the desire was for these to "match" the melodies of the ordinary, the creators of the OCM would have had to provide such melodies, and they did not. That would have been very jarring: 18-plus melodies being lavished on a newly invented feature, not to mention the hubris of implying that this novelty is of the same significance as texts which have been sung for 1,000 years or more.

One thing to note, there are two options given for these priest parts in OCM, a simple and a more ornate version, but otherwise basically the same tone: that is why the response is the same either way. I don't know that it is permissible to change the tones which the priest uses, I believe he is bound to use what is in the books. So for a chanted response, you are stuck with what is in the books.

Once you do something different, you are singing a composed setting, not chant. In this case it amounts to encouraging the congregation to consider Mass VIII as not being chant! (As being just another one of those modern Mass settings with a matching memorial acclamation and amen.)

Sidebar warning:

deep-sixed the G.A. about four years ago and haven't looked back.)

When I see this response called the Great Amen, I can't help thinking of a parish simply eons ago which sang "Ay-men; Ay-men; Ay-men; Ay-men;Ay-men; Alleluia" [rinse. repeat] by Joe Wise?

Firstly, I agree very much with the sentiments expressed above...

With regards to this offering itself, I personally find the setting of the "mortem Tuam" to be very awkward.

Beginning with the first neum of "Domine," there is trouble, because repeating that do, with Mass VIII in mind, is very unnatural and confusing.
"Domine" would either need to begin on re-fa, or just cut out the do-re and begin on fa (possibly even being syllabic, just to show that the change is intentional, and not trying to be the Agnus Dei).

From there, it feels odd to jump backwards to the "melody" of the +Benedictus, when you're coming off an introductory phrase belonging to the Agnus Dei... it just doesn't flow from one melodic idea to the other, when each is so obviously belonging to something else.

I would suggest that you continue working on it---even if you never end up using it at Mass, it is still good as an exercise to hone one's skill in the composition and adaptation of plainchant.

@ghmus7, contrary to what you say above, we have indeed been discussing the idea in the context of the OF, and the arguments are in no way specific to the Missa de Angelis.

A comment, I can't remember when I heard it, but in my memory I think maybe it was David Hughes, who observed that whereas historically the Mass ordinary set by composers was:

Kyrie
Gloria
Credo
Sanctus-Benedictus
Agnus Dei
[possibly Deo gratias]

The stereotypical modern composer instead has usually set:

Lord have mercy
Glory to God
[No Creed]
Holy, Holy, Holy
Memorial Acclamation
Great Amen
Lamb of God

If the absurdity of this is not readily apparent, that is only because this is what so many parishes have become used to over the past fifty years or so. The chants provided by the novus ordo Missal/OCM/Graduale do not follow this trend, and rightly so.

In the USA, singing the Creed was actively discouraged in non-legislative guidance and even 2007's "Sing to The Lord" provided:

170. The Creed is said by the entire assembly. Because it is an expression of faith by “the
whole gathered people,” the participation of all present should be carefully safeguarded,
whether it is said or sung. “If it is sung, it is begun by the Priest or, if this is appropriate, by a cantor or by the choir. It is sung, however, either by all together or by the people alternating with the choir.” The use of a congregational refrain may be helpful in this regard.

And GIRM (2011-USA), after STTL:

68. The Creed is to be sung or said by the Priest together with the people on Sundays and Solemnities. It may be said also at particular celebrations of a more solemn character. If it is sung, it is intoned by the Priest or, if appropriate, by a cantor or by the choir. It is then sung either by everybody together or by the people alternating with the choir. If it is not sung, it is to be recited by everybody together or by two choirs responding one to the other.

The Amen at the end of every Gloria* and Creed, in both GR and GS, is more elaborate than that given for the EP , I would say at least the elaboration of Gloria XV is warranted by the supreme importance of the EP. Acknowledging that it must match the sobriety of the chant of the prayer itself.
*(one exception, the more hispano in GS)

A refrain-based Credo might be one of the worst ideas I've ever heard.

And Mozart's use of the same idea is decidedly inferior to his treatment of the same text in the same summer (K. 194). At least the point in Mozart was thematic unity, despite being done better by other examples of his work. In a congregational setting, all it does is reduce the Creed to something simplistic and ultimately meaningless.

Either of the attached Amens work perfectly with Missa de Angelis

You can "ignore" the "great" Amen by simply reciting it or using the Gregorian formulae, neither of which harm the rite or its prescriptions in any ways and will, at worst, damage the egos of composers who composed them and didn't know any better.

As for the MA, I use the chant settings as lightly accompanied as possible, usually unison only.

The problem with the Great Amen as it is set by composers these days (Shenk, Proulx, Joncas, Hass, et al.) is that it intrudes. It isn't a grand utterance, like the Gloria or Sanctus, only one word (and that usually repeated ad infinitum). Has anyone really listened to how odd it sounds for a priest to chant the Missal tone for the Doxology, outlining a pentatonic scale, and then having the organ and people (plus brass or whatever) blast out the Great Amen in one of the usual settings?

I never thought anything of it, myself, until after the Missal Translation changed and we were using the Missal tone for the Mystery of Faith (Save us, Savior...), I began to feel that the Danish Amen or the Shenk "Mass of St. Therese" Amen that we had been using before seemed out of place with the Mystery of Faith from the Missal (I didn't like any of the re-adaptations for any of those Masses, and the pastor wanted to use only form C): With the Old ICEL/ICET version, the Acclamation and Amen (Christ has died/Amen) were always a set. Now that we have only been doing the simple chant Amen (and the Missal tone Acclamaiton), I always find that the through-composed Amens connected with various Mass settings seem out of place, too grand. Too much amPHASis on the wrong thing.

Sometimes the most important parts of liturgy are shown by their simplicity. After all, the Gospel Tone is the simpler than the Epistle Tone; The Canon (including the words of institution) used to be said silently; The Orations are all set to simple tones, but the Graduals are highly florid. Personally, because of the importance of the Eucharistic Prayer, I feel that the Amen after the doxology should be simple.

(This, of course, is not to denigrate the work of the composer who started this thread.)

Well, that is just not how it is. And I think that its significance is thus different than you would suppose.

The "Great Amen" does seem to get a lot of hype in novus ordo circles of thought. I am not sure to what extent the documents call for this. But the chant does not play this game. You can say that those responsible for preparing the official chant for the novus ordo failed in not creating something special to go along with this great new emphasis that was in the air; I say that they had sense enough to leave things be.

A thought:

If textually this is the conclusion of the Canon, yet musically it is a beginning, and preparatory to what follows.