The tuning of keyboard instruments has always been a matter of debate. The 'comma' (interval between two tones derived from one common tone by stacking either 7 octaves or 12 fifths on it) must be handled in some way. Johann Sebastian Bach's Well-Tempered Clavier (das Wohltemperierte Klavier) was a definite appreciation of chromatism in music, and it has been assumed that the title referred to equal temperament, the standard modern keyboard tuning. Current scholars think he meant something different, possibly a range of similar temperaments, perhaps even altered slightly in practice from piece to piece, or a single specific "well-tempered" solution for all purposes. More details can be found here.
On August 4, 2009, Nicolas Davidenko and I (Kees Swenne) have done an experiment to get a better idea of the relevance of the tuning, with as an example Praeludium 1, BWV 846. We have tuned a Neupert 'Silbermann' spinet, kindly provided by Mrs. Saar Hack, in the Kirnberger 2 temperament. This is not difficult to realize: start with tuning Eb with a tuning device, and proceed by tuning 5 pure fifths, Eb-Bb-F-C-G-D, and 5 pure fourths, Eb-Ab-Db-Gb-B-E. The comma is resolved when tuning the A: interval D-A is somewhat smaller than a pure fifth, and interval E-A is somewhat larger than a pure fourth. This tuning hence gives many pure fifths and three pure thirds: C-E, G-B and D-F#, not so bad for the C major prelude.
After having recorded the prelude in Kirnberger 2 tuning, we retuned the spinet in equal temperament and recorded the prelude again. Listen to the result. Can you hear the difference? We post the files here as A and B, not necessarily in the order Kirnberger tuning - equally termperd. Listen to the tuning only, don't listen to differences in the interpretation (either of us has recorded one file). Both files are in WAV format and have a size of about 25 Mbyte.
Listen to A
Listen to B