I'd like to offer you a completely different perspective since I'm not a Twitter user:
First, I'd suggest you improve your web copy. After I read your home page, I wondered: What's the benefit? Consider your copy from the perspective of a prospect: Why not simply make a public tweet about the new album or the new article? Where's the advantage when using your service?
Your features -- that your service makes it easier to send out many direct messages, to target different groups, and to track results -- are valuable, only, if a prospect understands the benefit.
Second, I wonder whether musicians and journalists are really your best target audiences?
The web is full of web marketeers and they are used to create campaigns. Almost by definition, they always promote something or know someone who does. If you think about their typical tool box (SEO, SEM, etc.), it should be easy to find them: search for appropriate forums, Facebook groups, conferences, journals, newsletters, and, of course, Twitter accounts.
Some of them even promote themselves to other marketeers. They are also more likely to promote your service because many of them need new content for their blogs. So use a Blog search engine to find the most influential bloggers. Probably, they are also more used to the idea of being contacted for promotion purposes.
However, if you like to target musicians: why not reach out to others who already sell something to musicians? For example, companies producing music equipment, journals and web sites reviewing music equipment or the music industry, internet forums about hard disc recording, etc. Get in contact and try to find a way to cooperate or research advertising opportunities. Some of them may already have a Twitter account and many of their followers are likely to be musicians, so try to have them tweet about your service.
Additionally, you should be able to build a list of potential contacts quite easily. Musicians need to promote themselves and sell their music. There are already quite a few web sites who try to help them do that. Think Amie Street and similar services for independents. Get the band names, search the web or MySpace for their home page and then contact them by E-Mail.
The same line of thought could be used for journalists.
Hope that helps.