A reader asked me to clarify what I had meant about the "corrupt Solesmes" chant as contained in the Liber Usualis. Since music is not my area I referred the question to an expert in sacred music, whose insightful response, for which I am very grateful, is here:
"It is a complex issue. Styles of Plainsong changed many times – often to do with fashion. Nicholas Wiseman in England pushed for uniformity and Italian pronunciations in England well before they were imposed by Pius X. The Ultramontane goal was to do away with regional – especially Gallic - variations in chant and centralise everything by coming up with a single version to be exclusively used.
"The Liber Usualis is basically based on a mediaeval style as imagined by monks who could never have known how plainsong was sung in mediaeval times. They worked on up to forty variations of a given melody and constructed a new melody on a democratic basis; so if eight of their models had certain notes at a certain part and twelve of their models had something different they would incorporate the latter.
"Performing plainsong at an almost funerial pace started in the Renaissance, as did slowings and speedings to give a word-emphasis. The slow paces, once they had been officially adopted by Solemnes, stuck.
"Personally, much plainsong – the way it is now - I find unbeautiful and depressing. I think monks probably always sang it at a slow pace because there was no rush but that was not the case outside where a lighter and faster pace was common.
"This has not really answered your question but the extracts from the following book might be of help:"
On a personal note, I might also recommend this channel on YouTube, which has a trove of chant traditions, both Eastern and Western.