I have a powered silver box which is the one I think your are talking about that provides a line out signal, but it doesn't power my guitar. In fact the battery goes flat quicker when I am using that. While that box provides a much cleaner output for the acoustic emulations the electric models don't drive my tube amp as well as they do when I simply plug in a regular guitar cord.
I am guessing there may have been issues with the Variaxe 300 which may have been why Line 6 stopped making it. But I do like that guitar a lot.