A friend of mine found the following as part of his job at the library,
https://www.digitale-sammlungen.de/de/view/bsb11175114?page=200,201

See items 6 and 7 on the righthand page,
Transcription and translation (not mine)!,
§ 6 (...)Daselb sein rastbetlin darauff er sitzend gesungen, und seine gaissel, mit welcher er den Singer knaben gedrowet, in billichen ehren, sammt dem rechten vsprunglichen Antiphen buech biß auff den heutigen tag bewahret werden

(Gregory founded two houses for his choir school, one under the steps of St Peter's, the other under the buildings of the Patriarchate of the Lateran.) There, his sofa, on which seated he was singing, and his scourge, with which he threatened the choir boys, together with the real original Antiphoner, have been kept in appropriate honour until our days.

§ 7: Diese liebliche Gesang haben vnder andern Völckern Europae, auch die Teutschen und Frantzosen wol lernen und offt widerlenen können: aber auß leichtfertig jrer hertzen, oder aber auß wilder angeborner Natur haben sie dem Gregorianischen Gesang jre aigne gesang eingemischet, vnd derhalben die rechten nit können halten. Diese berggrose cörper donnern mit jren Stimmen hoch, vnd lassen der fürgeschriebner Gesang liebligkeit nit schallen. Die wilde art der versoffnen hälsen, wenn sie die Stimmen biegen vnd ein linden thon machen wöllen, bricht für, kracht wie ein gefrorner oder vngeschmirbter wagen auff dem plaster: solt der zuhörer hertzen erwaichen, vnd aber sie erbitters.

Amongst the peoples of Europe, also the German and French have been able to learn, and often learn again, this lovely chant. However, because of the levity of their hearts, or of their inborn savage nature, they mixed their own chant into Gregorian chant, and thus cannot keep it right. These bodies as vast as mountains thunder high up with their voices, and do not sound the prescripted loveliness of the chant. When they should inflect the voices and make a soft tone, the savage manner of these boozy throats breaks out, bangs like a frozen or ungreased carriage on a cobbled street, it should soften the hearts of the listeners but embitters them.