Hmm. Either Ron Pattinson's assertion that Kottbusser was a sour beer is a little off, or else the revival of this style does not require that it is soured. In my thinking, this probably really was a Lacto soured beer and predecessor of Berliner weisse. Since it's all ancient history, I suppose the style today is whatever we imagine it to be. For the above recipe, it's really somewhat of an imperial Kolsch. If you're not afraid of Lacto, I would suggest adding some to at least a portion of the batch, to see how that turns out.