Metal seems to have greater potential as a meme than classical music. Musical tropes are often used as examples of memes and as the starting point for discussing exactly what a meme is. (e.g., is Beethoven's Fifth Symphony a meme, or just the four-note da-da-da-dum that literate Westerners would recognize?)
The key is in how the meme is reproduced process. In classical music, you put on a CD or go to a concert, and for two days you're walking around humming "da-da-da-dum" - but does anybody else hear you and get infected? No. Of course, the same is true of that new metal CD you just bought. The difference is that someone who listens to a lot of metal is far more likely to pick up an instrument and make their own metal. When was the last time someone who listened to Bartok ran out to buy a violin? You could make an argument that this happens because people who listen to metal are on average younger and more impressionable, partly because of the parts of the brain that metal affects. As an aside, at 35, I'm sad to say I'm developing an immunity to its charms, though I've fought this seeming inevitability for several years now.
Fear not, classical music enthusiasts. Classical music isn't going anywhere, at least right away, because it has prestige value, and a wealthier (older) audience, and an established academy to produce musicians. But the market share of such a bottlenecked genre is necessarily vulnerable to the expansion of self-driven, home-grown, grass roots genres. To continue the tradition of meme enthusiasts of extending extend the analogy too far, the reproduction of classical music is eusocial, and metal is, if not viral, then bacterial.