The theorbo is a sort of big brother to the lute, with a set of long, unfretted bass strings in addition to the fretted courses. This gives the instrument a wonderful, rich mellow sound which is beautifully evoked and captured in these recordings (from 1992 and 1999). Montailhet is an excellent theorbist and the music, too, is a delight.
Robert Shaw's reading of the B Minor Mass is, in one sense at least, just what one would expect: sober and purposeful, beautifully shaped (Shaw is a master architect), it centers on the chorus. Like all of Shaw's choruses, the Atlanta group has that trademark richness of body and blend, and it sings with utter unanimity as though it were one great voice. Shaw opts for marginally broader tempos than those found in most period-instrument performances but is nowhere near as glacial as some interpreters.
That Bach's motets are not his most popular sacred choral works is easily explainable. When asked to choose between the Passions' spiritual drama, the Mass' magnificent architecture, the cantatas' infinite diversity, or the motets' inward piety, most listeners will more likely choose any of the former before the latter. Still, for concentrated intensity, it is hard to surpass the motets, particularly in performances as superlative as these by the Nederlands Kamerkoor under Peter Dijkstra.
Here's a collection of Bach pieces where the performances fit together exceptionally well. The basic sound is that of Canada's veteran Baroque ensemble Tafelmusik under violinist and director Jeanne Lamon: smooth, bright, French in its light seductiveness. Sample the almost accentless Rondeau from the Suite in A minor, a violin-and-strings transcription (and according to musicologist Joshua Rifkin a reconstruction of an original version) of the Suite in B minor for orchestra, BWV 1067; you couldn't call it gutsy, but the degree of control and consistency is impressive.