All In A Name : Going off-topic when you're a niche community

I would suggest creating a landing page for your website, which clearly details that Sounds From the Grave is a website for metal, but also features another section for Classical. Introduce ways to navigate to other parts of the website, like having a button to access the Classical part, and have a short description saying something like...

Enter the classical zone, where you can browse a large variety of classical music that fits your more refined tastes.

Sounds From the Grave doesn't necessarily have to be "music from death," it can also be music that was created by now-deceased artists, which is what classical music really is, isn't it? Bring that vibe forward, showcasing the music of old, but not forgotten artists. This name ambiguity can serve you well, and you can refine what Sounds From the Grave really means.

Classical can then have its own landing page, or maybe just an informational header, that briefly explains (in better detail) what that section of the site is about.

Stack Exchange does this as well. When you first access Stack Overflow (you may want to log out to see it in its glory,) it completely explains what Stack Overflow is, and highlights its unique Q&A content. Since Stack Overflow is the hallmark part of the network, it gets quite the introduction. Another site on the Stack Exchange network, Science Fiction and Fantasy, has a simple header that briefly introduces you to the site, and allows you to join it with the press of a button.

Notice that the art style is entirely different to what Stack Overflow normally presents. The color changes very clearly indicate to the user that they're on another site entirely, and just as they're browsing the art, they'll come across that header, which explains what the site is about and how it's used.

If you want to make your classical audience not feel like a sideline, make sure that users who visit that part of the page feel like they're a part of the main site, but are their own little community. Enable them as much as you can, but make sure you're not detracting from your mainstream, death metal audience. You can do this by ensuring that the death metal lovers' discourse or music doesn't flow over into Classical's site, and the opposite doesn't happen either.

Make sure your presence, as the website owner, is known on both parts of the site, so that both of the communities that are there feel welcome by the owner him/herself.