I am curious about the circumstances of the composers mentioned in that thread as well as the adoption of their compositions. For example, what motivated those composers to adapt ecclesiastical hymns written in the New Method to Slavonic?
From what I have read, it seems in the medieval times there were two musical traditions in Bulgaria. First, there are many preserved Byzantine musical manuscripts in Greek language. Apparently the chanters had to learn the music using the original Byzantine Greek texts and notation and then to adapt it to the Slavonic text but without musical notation. Maybe in some monasteries only Greek was used (from the biographies of the saints one can conclude that many of the medieval monasteries were international, it wasn't unusual for Greek and Romanian monks to inhabit Bulgarian monasteries, or for Bulgarian monks to go in Byzantine monasteries, etc.).
The second musical tradition was purely Slavonic. Little is known about this music, there are few preserved musical texts in a very primitive notation. It seems this tradition was more popular in the western part of Bulgaria, where the monasteries (for example the Rila-monastery) were occupied only by Bulgarians and maybe other Slavs. My personal opinion (not an opinion of a professional musicologist) is that this music was not Bulgarian, but the music used by all Balkan Slavs (Bulgarians and Serbians). During the Ottoman rule this Slavonic chant disappeared from Bulgaria, it was however preserved in Serbia. This can explain why the melodies of some apolitikia for Bulgarian saints sound as melodies of the traditional Serbian chant.
During the Ottoman rule the Byzantine chant became the only known chant in Bulgaria (the proper terminology was Eastern ecclesiastical chant). The first Slavonic adaptations and original compositions using the old Byzantine notation are from the second half of 18th century (after Petros Lampadarios) and the main centres for these adaptations were Rila monastery and Mount Athos. At that time it was customary for the Rila monastery to send young prospective monks in Mount Athos to learn music, who would return after several years of education. By the way, metropolitan Seraphim of Bosnia, who is mentioned in
this post ended his life in Rila monastery. At the beginning of 19th century there was already a lot of Slavonic musical repertoire in old notation. Most certainly, however, this notation was known only in the big monasteries (Rila monastery and the Slavonic monasteries on Mount Athos) so at that time the Slavonic adaptations and compositions had very little impact.
The real boom of the use of the Byzantine musical notation for chant in Slavonic came only with the invention of the new notation. I don't know why the Rila monastery didn't print its Slavonic compositions but the enthusiasm among the people was so great that these compositions became very popular in the form of handwritten manuscripts. Angel Sevlievetz was the first who printed these compositions but he printed his books without permission from the monastery. A friend of mine asked a monk of Rila monastery to loan the books of Angel Sevlievetz (in order to copy them) and he was answered that they don't have these books because of this.