The fourth cycle of cantatas, which constituted Vols 19 and 20, marked the end of Bach’s systematic liturgical ambitions. That Bach abandoned the project is emblematic of a composer who had decided by the late 1720s that cantatas had run their course as a primary creative means. Instead, the last two decades in Leipzig reveal Bach’s almost exclusive concern with keyboard music, although the final pieces in Koopman’s monumental vocal jigsaw present the last occasional choral works. Good housekeeping also prevails here with various musical appendices, including the spurious No 143 (both Bachian and under par at once), the supremely poignant “antico” funeral motet (No 118), the Gloria (No 191) and miscellany.

True to the disparate scheduling of sessions, these performances were taped between 1999 and 2005. If a consequent sense of a voyage of discovery and incremental achievement is missing, as it has been throughout, there is still plenty to admire. The instrumental playing is often a class apart from rival sets, certainly when one feels that enough time was available and care taken. When it was, textural vibrancy in the strings, the winsome and delicious strains of flautist Wilbert Hazelzet (the flute figures significantly in “galant” conceit from the cantatas of the mid-1720s onwards), Katharina Spreckelsen’s beautifully judged and blended oboe sound and many other fine players contribute to movements of exceptional colour and purpose. One thinks immediately, in Vol 21, of the delicate filigree of the opening chorus and arias in No 177, a chorale cantata of radiant efficacy where Sandrine Piau and Christophe Prégardien buck the trend of pot-luck solo singing with exquisitely delivered joie de vivre – confirmed in the latter’s aria by the cheerful clucking of an obbligato bassoon.

The distinction between sacred and secular cantatas is significantly blurred in Bach’s later years especially, and secular material is revisited for sacred purpose (and vice versa), as in the two performances of BWV30: the church cantata unabashedly recycling the “welcome” for a new heir to Schloss Wiederau to the prophetic arrival of John the Baptist. The free interchange is also evident in the four so-called Lutheran Masses (Kyrie and Gloria only), where Bach reworked old cantata movements with customary brilliance. Koopman’s readings appear considerably less inhabited or refined (the choral singing is not as good as usual) than Philippe Herreweghe’s Collegium Vocale who produce the most fragrant “period” readings to date.

Indeed, it is the corporate ensemble of the Amsterdammers, conveyed in the contrapuntal gymnastics of Ein feste Burg, the wedding cantatas (Nos 97 and 195, both given dazzling performances) and No 30, which allows the best performances to emerge. When the Amsterdam Baroque Choir lead the line at their glowing best, the results suggest that over-directed Koopman-Bach is often rather less successful than when the music is left to play itself. The ubiquitous Wachet auf is too manipulated (Werner and Ristenpart – Nonesuch, nla – are the answer here) and O ewiges Feuer (No 34) is mundane compared with Richter’s and Gardiner’s triumphal readings.

Again, certain singers carry the day – especially Sandrine Piau and James Gilchrist, the latter profoundly telling in “Ich traue seiner Gnaden” from No 97. One should give special credit to the stalwart of the series, the bass Klaus Mertens, capable of both understanding and beauty, if not immune from the ethos of pragmatism over enlightenment which has prevented this intermittently outstanding series from realising the promising portents of 1995.