Or just let it die. I'm not sure I buy your reasoning for deleting almost every subforum that could have targeted a niche. I don't think anyone will be drawn to the message boards when it's just some vague "community" that has nothing to distinguish it from other message boards except for how slow and spammy it is considering that it's attached to a fairly well-read music criticism site.
What you need to focus on is making people actually want to post, because obviously now hardly anybody does. This makes it hard for you to find a mod who would actually consistently clean things up, and also negatively feeds back into itself by discouraging people from joining what looks like a dead community.
The way I see it, two things make people want to contribute to an online forum 1) social affirmation of their contributions (via rep and up-voting and things like that), which is annoying, and 2) making it inherently fun to take part in the community. That's why I suggested writing and music competitions. Some online games are also fun (I think we used to play Miniclip golf). I don't know how you could actually encourage this kind of thing, but maybe if you really care you should think of a way.
It's likely that it's hopeless and the forums will never be popular because they're slow, they don't target anyone in particular, the review site itself isn't very renowned or innovative, and we don't have the up-voting system that makes sad needy people flock to sites like Reddit. Maybe that's okay.