Speaking of which ...

... 20th Century Music--What Went Wrong? (Hat tip, Dave Lull.)

The Kingsley Amis quote demonstrates that even very bright people can say stupid things. Plenty of good, tuneful music was written during the 20th century, and not just by Rachmaninoff, Sibelius and Richard Strauss. Poulenc, Martinu and Barber wrote plenty. And even if it is more dissonant than Sir Kingsley would have liked, Bartok's music and Walton's, and even that by Ives and Ruggles is worth a listen.
That said, the century did produce a lot of clinkers (but so did every other century; only we don't have theirs forced on us usually). A few years ago I sat through a performance of Schoenberg's Gurrelieder - not one of his 12-tone pieces - and just found it bombastic, meandering and boring. Verklaerte Nacht - Schoenberg at his most tonal - is just second-rate Wagner (of course I happen to think that most of Wagner is second-rate, too).
The real problem in the 20th century was the notion that there can be progress in art. There can't. However much one may like Picasso (I prefer Braque), his work does not represent any sort of progress over, say, Botticelli. Nor would Picasso himself have claimed that it did. Apart from the development of the symphony orchestra, music has not "progressed" beyond Bach (who wrote some tone rows himself, actually, or so a musician friend of mine assures me).
The only way you can give the illusion of "progress" in the arts is by means of "stylistic" and other extravagances. Nonrepresentational art and atonal music permit the talentless to pretend to originality (another dubious artistic value). But it is hard to write good free verse, hard to paint a good nonrepresentational painting, and hard to write good atonal music - but it can and has been done - by poets, painters, and composers of genuine talent and skill. Unfortunately, it is all too easy to do all of these things quite badly and yet, by dint of skillful PR, manage to make a career of it.
I am listening right now to Arthur Honneger's third symphony, which is going to be played at a Philadelphia Orchestra concert later this season. Kingsley Amis would probably not have liked it. It has some passages of unresolved dissonance. It is, however, quite a good piece and I'm glad I'm going to be able to hear it in concert. The best thing conductors can do for music and audiences is to play more music by good composers whose music doesn't get played enough - and to exercise some courage and judgment by not playing crap commissioned by some tone deaf foundation committee.