The Preserving Machine is generally perceived as the time-honoured tale of a scientist whose invention has unforeseen consequences, and/or a satire on the fragility of human civilization. Of course it's both these things, but if either of them had been Dick's primary intention when writing it, the story would have had less emphasis on the composers and their musical works, and more emphasis on the machine itself and the way the creatures get out of hand. As it is, I can't help thinking that Dick's original intention was to make a point about the relative resilience of different composers' works, and that he came up with the story simply as a vehicle for symbolizing that point.
One reason for this view is that the story, taken at face value, is pretty ludicrous. At the time it was written, audio recordings in the form of 78 rpm records were well established (in fact PKD's first work experience was as an assistant in a record store). But the preserving machine doesn't use audio recordings, it uses printed scores. What use would those be to future civilizations? They could understand audio recordings of music, but dots and lines on paper would be meaningless to them! On the other hand, to a classical music buff, the score is the true work of art -- not any particular interpretation of it. This was obviously Dick's view at the time the story was written.
The real significance of the story is in the description of the forms taken by the works of different composers. The Mozart work (not a popular piece like Eine Kleine Nachtmusik or the overture to The Marriage of Figaro, but the much more highbrow String Quintet no. 4) takes the form of a miniature peacock-like bird... presumably symbolizing beauty and delicacy. Bach's Forty-Eight Preludes and Fugues come out as small spherical bugs, presumably representing mathematical precision (although their geometric perfection is soon marred by the development of poisonous spines). Brahms, that most conservative of composers, is represented by a low, flat, many-legged centipede... the ultimate in stability and unflappability. The Schubert animal is a sheep-like creature, playful and immature, which is soon torn apart by the Wagner animal -- a fierce, wolf-like creature.
The most interesting creation, and the one that I think the story is really all about, is the Beethoven beetle. Of course, beetles symbolize resilience and survivability, which is the whole object of the exercise... so there's a hint there already that Beethoven's works are the "best" in this respect. But more significantly, the Beethoven beetle is the only creature that doesn't change itself in order to adapt to its environment. At the very end of the story, the beetle has built a little mud house for itself... quite a sophisticated one, since the beetle is described as "snapping the door firmly shut behind it". In other words, the Beethoven beetle has adapted the environment to suit itself, rather than the other way around!
Postscript (December 2013): Here is a short video I put together on the same theme as this blog post...