The Art of the Fugue, great summit of achievement in contrapuntal writing, timeless enigma of Western art, purest expression of music's foundational genius (see the apt, if oft-disputed epigram found on the cover of the 1746 manuscript of Contrapunctuses 1-4 : "Wie gefällt dir mir jetzt?"*)...can be a bit problematic in performance. The probability that the average human brain is processing all that dense counterpoint, on roughly the same theme, in the same key, in any sort of pleasurable or enlightening manner, rapidly approaches zero as the contrapunctuses march on. So one needs to find ways to keep it fresh, and the varied configurations the Bach Consort brought to bear Sunday--including solo organ, two harpsichords, and a period string quartet of violin, tenor and treble viol, and violone--provided enough variety that one happily never succumbed to the dreaded fugue overdose.
That said, there is surely a case to be made that period strings are not an imperative when dealing with the Art of the Fugue. No doubt the quartet produced an appealing sound, especially where organ doubled certain parts to bring out the fugue themes, and Andrew Fouts (Bach Consort's concertmaster) had some of the most exhilarating playing of the afternoon in parts that lend themselves especially well to the violin, such as the syncopated melody of Contrapunctus II. But the violin just seemed to highlight how often the counterpoint ended up muddled in the bottom three strings, both on account of the imprecision in performance that tends to crop up with these instruments and the fuzzier textures they produce. It's hard to listen to something like the Emerson Quartet performance below and not suspect something is missed about this work when precision is sacrificed.
No such complaints about the use of harpsichord though (the two onstage were played here by Bach Consort leader J. Reilly Lewis and Scott Detra). I'm all for Bach on the piano, but listening to the Aimard recording as I write this, I am reminded that piano in this work specifically is perhaps one layer of information too much. The dynamic choices seem falser than they do elsewhere, more futile, and the whole magnificent edifice sort of degenerates into sounding new agey music. The austerity of the harpsichord, on the other hand, keeps us riveted to the score without distraction.
*i.e. "How you like me now?"