It is a good idea to take the literature of music into account when trying to make a determination regarding historical temperaments. Take the recent discussion of Brahms' preferred temperament in the December 2013 journal. After reading the various points of view I would like to add some of my own observations. The tempering of intervals at the is primarily done within the range of the human voice. For keyboard people that means the middle of the keyboard. At the extremes are what the tuners like to call "stretched octaves' and "enharmonicity" and what really happens is the ever nearing perfect pythagorean fifth. A pure fifth results in a stretched octave in either direction, up or down at the keyboard. Thus neither equal temperament nor well-temperaments can be strictly applied at the high and low ranges, nor are they practically applied because tuners and players would find them to be objectionable due to the non-stretch of those octaves. This further explains why it is said that few keyboard players can tell the difference between an equal temperament and an unequal one. As are keyboardists they're just happy to have something reasonably in tune. Furthermore, if they have been listening to wide thirds all their lives they are less likely to be disturbed or even notice slight variance there.
The tradition of vocal music, modes, and the importance of key and mood ( derived from mode) was already in existence for centuries before the onset of keyboard instruments. By the 18th century it became apparent that some kind or accommodation in tuning these newer arrivals would have to be made.Thus began the circulating temperaments or well-temperaments as part of the musical discussion. Generally these tended to favor the keys most used in the interest of harmony, the evolving musical understanding of freezing the succession of lines of music in time as a means of structure in music. These keyboard temperaments were mostly biased in favor of more harmonious sounds in the keys closest to C major in the circle of fifths.The characteristic and general tendency was then to have slower beating thirds in the nearer keys and faster beating ones in the more remote keys from C. Another characteristic was the use of pure fifths (pythaogorean fifths) among the sharp keys which were tonic and dominant in the keys farthest removed from C. The pay-off was some sweeter thirds and some pure fifths and so it was considered as worth making some sacrifices to attain this.
Composers such as Bach realized both the limitations and the possibilities for expression in unequal temperaments, also called well-temperaments or circulating temperaments. Give something up and get something back as a result: this is the tempering, tampering, or tweaking of the intervals within the octave. Not surprisingly the varied styles of composition in the Well-Tempered Clavier are prescribed by what the home key is and the harmonic and melodic advantages an disadvantages afforded in each key by unequal temperament. This became established tradition and there is no doubt that all the great musicians of 19th century Europe were familiar with the Well-Tempered Clavier"and the meaning and significance of the various tonalities. It seems unlikely they would just chuck it away in favor of some kind of new tuning standardization espoused by the "new professionals" the tuners, who did not exist in force prior to the time of someone like Nanette Streicher (ne. Stein) in Vienna
If Equal Temperament became an expedient for people charged with keeping keyboard instruments in tune throughout the 19th century as has been alleged, one still must consider the importance of all music literature not specifically contrived as keyboard literature. Music historians are generally in agreement as to why certain pieces of music are expressed in certain tonalities. With regard to the orchestra we have string instruments set up essentially for Pythagorean tuning by fifths, and brass and woodwind instruments set up to be most expressive in very definite tonalities. Trumpets set up for D major express their own triumph in the Gloria of the B minor mass of Bach, and Mozart's Don Giovanni gets dragged off to hell in the daemonic key of D minor. If you want a keyboard example Examine the classical composers see for yourself why something like the Waldstein Sonata opus 53 of Beethoven can only be expressed in the wide-open key of C major. Take his F sharp major piano sonata opus 78 into account as well for contrast. Its questionable whether such notions could arise in an equal-tempered scheme. As regards the orchestra can we assume a rising sense of discord as we proceed from the symphonies of Mozart, Haydn and Beethoven to those of say Mahler and Bruckner? This is the period in which equal temperament is said to have been on the rise. Does the importance and meaning of tonality take on less importance as the 19th century progresses?
Since it seems that the piano starts to mount a take-over as the dominant player in 19th century European music we might as well consider the key selection for chamber music of Brahms for example, the first trio in B major. I have heard the work played in a way that sounded like some tension was being expressed between string players playing in Pythagorean tuning and pianists playing in equal temperament. I have been told that the very best "professional" chamber players can make the adjustment, but it has to be all string players making it since the piano is essentially fixed in its pitch. When a piano takes on an orchestra it is more like a war than some of the mild tension of chamber music. Not only is it a power trip for the piano to be able to play loud enough to give the orchestra a run for its money, but there is the certainty that equal temperament is trying to overpower the Pythagorean tuning of the orchestra.
Consider the possibility that if the piano becomes a too dominant in the music literature in the way that it is tuned then some very long-standing an ancient principles of music may fall into neglect and even disregard. In reaction there have been repeated attempts to throw off the notion of standardization from the musings of someone like Terry Riley In C to the blatant prepared piano of John Cage. Throw the bi-tonality of Charles Ives into the mix and appreciate the discord! We might concede that some mistakes have been made in the 19th and 20th centuries and that tuners today who know what they're doing can do what they like and achieve quite acceptable results without having to resort to a "one size fits all" standard. It has already been pointed out that so few can tell the difference anyway.