I might have mentioned this before, but if you've ever heard "Gregorian Chants" and you haven't heard le Ensemble Organum you are doing yourself a disservice.
Led by Marcel Perez, le Ensemble Organum has spanned a number of styles. Occasionally they dabble in historical reconstruction, such as the Old Roman Chants. The results, by the way are fantastic. Meticulously researched, the ensemble married Greek drones (ison) to Western liturgical chants; it probably was that way at some point.
Did you notice the drone? Sounds quite a bit different that the Gregorian Chants you usually hear.
They perform quite a wide variety of styles within the liturgical chanting world, though sometimes their albums are expensive or hard to find. Always quality stuff if you ask me, which you did by reading this blog.