N900 has a headphone output, and, as a result, headphone to line levels should be done externally.

However, in my experience N900 has a standard driver and should be usable with a line input at low volume. Are you using a balanced input on the other side? What exactly is the problem you are having?

Also, "line" level is not standard, at least not across the globe. Since you didn't set your location in your profile, can't even guess what line level you are referring to.

If you want to hook it up to a PC or audio system line in, then low volume should do the trick. Also, have a look at the TV output, though I don't expect significant differences.