CLASSICAL Janacek, Leos, ''Glagolitic Mass'' (two discs): Gabriela Benackova-Capova, soprano; Eva Randova, mezzo-soprano; Vilem Pribyl, tenor; Sergej Kopchak, bass. Czech Philharmonic Choir, Brno State Philharmonic, Frantisek Jilek, conductor. (Pro Arte/Supraphon Pal-1060.) Also, Felicity Palmer, soprano; Ameral Gunson, mezzo-soprano; John Mitchinson, tenor; Malcolm King, bass. City of Birmingham Symphony Orchestra and Chorus, Simon Rattle, conductor. (EMI-Angel DS-37847 - Digital.) - Janacek's ''Glagolitic (or ''Slavonic'') Mass,'' is one of the most original devotional compositions of the century. His language is unique; his use of orchestral forces creates a fascinating blend of primitive savagery and opulent beauty. Jagged rhythms, slashing brass outbursts, and melting strings all combine to create a sound world unlike any other. That sense of the primitive must be exploited: To smooth out the sounds and the rhythms renders every climax ineffectual and pointless. It is not surprising that Czechs understand this, but it is surprising that most British conductors (Sir Charles Makerras being the one exception) do not. The gifted young Simon Rattle reduces this blazing testament to a rather tame oratorio, and the singing is only passable. The work was recorded in an overly resonant hall, and the organ is too thick to allow for the sort of clarity of voicings that would make those important moments effective. The Supraphon release is far more to the point. The singing is uneven, but consistently expressive, even racked with the requisite emotion. Maestro Jilek knows that mere prettiness is not enough. There has to be a parallel ugliness, and his Brno forces get it. The pressing is quiet, the organ playing is splendid, and very Janacek. Nonetheless, the one who gets this piece right on all counts seems to me to be Rafael Kubelik on DG records (138 954). Even with non-Czech soloists, the performance is engrossing, and savagely haunting. It remains one of the most important recordings of a Janacek work in the catalog.