I first became aware of Ludwig van Beethoven's three equale for four trombones (WoO 30) during my student days, when I purchased a (vinyl) disc on which Jean-François Paillard had compiled a "historical anthology" of brass music, with Josquin des Prez at one extreme and André Jolivet at the other. (I think the recording was called Fanfares from the Sixteenth Century to the Present.) Beethoven was represented by the first of those equale. At the time all that really struck me was how little it sounded like any Beethoven I had previously heard; but, since my listening experience was pretty limited back in my freshman year, I did not give it much further thought until it showed up in that Brilliant Classics Gesamtwerk collection of Ludwig van Beethoven. By this time my listening experience had become far more extensive, but this music still had an uncanny uniqueness to it.
This prompted me to attempt a bit of research. About all that I could cull from my Thayer was that the music had been published in 1812. Grove Music Online was a bit more informative. First of all it reminded me of what I should have already known, which is that an equale is a composition in which all the voices are the same (or very similar), hence the "equality" of the four trombones. I also learned that Beethoven wrote these three short pieces for performance in Linz Cathedral on the Feast of All Souls, leading me to believe that he was interested in the solemnity of the sound of the trombone, perhaps inspired by the way Wolfgang Amadeus Mozart had featured it in his K. 626 setting of the requiem mass or, for that matter, the ritualistic elements of Die Zauberflöte (K. 620). However, the "constraint of equality" obliged Beethoven to come up with four-voice harmonies that were more "tightly packed" than we would encounter in more conventional ensembles, such as a string quartet. I think it is the way in which he approached this problem of "density" that made this music as unique as it is; and I do not think I have encountered any other Beethoven composition that works with such a "tight equality." Perhaps he was not particularly satisfied with the results; and, having fulfilled his obligation to Linz Cathedral, returned to other pursuits that interested him more. (1812 is also the year of his last violin sonata and may also be viewed as a "calm before the storm" in which he worked on his final round of piano sonatas, the ones that András Schiff will be playing in the last two recitals of his traversal of all of those sonatas.) So, while we should probably not read too much into these three little trombone pieces, they do have a way of sticking in the memory once they have been encountered!