I think you need some method of transforming the impedance of the speaker (~ 8ohms) to a source impedance that the mic pre would like to see (on the order of a couple hundred ohms). I think a passive DI box can do this, but I would try and find a transformer that could step up the impedance. You should also get a gain increase due to the transformer as well. Try something with a turns ratio of anywhere from 1:2 to 1:8.

Someone may want to check my answer above, I haven't had much caffiene yet...

Ian