What are the limitations of "fake" balanced audio cables?
I was looking for an audio transformer to convert the unbalanced into
  balanced signal


Why bother - if your inputs are balanced then there is no problem using them directly connected to your unbalanced pre-amp output. Use a balanced cable and make sure the shield is connected at the speaker end. Don't connect the shield at the pre-amp or both ends. This is to avoid ground hum or possibly ground currents unassociated with the audio.

A signal that is 6 dB weaker may not result in a signal-to-noise ratio degrading of 6 dB. It all depends on the noise and whether it is mainly differential or common-mode.


  What is the length that should not be exceeded with this solution?


This cannot be answered without knowledge of the prevailing noise and interference levels at your installation.