Florian,
It may not be a problem once you get your preamp back. In almost all preamps the volume control is the first thing in the circuit, and will attenuate the level to prevent it from overloading the input stage. There are some preamps that put the attenuator after the input stage, but they are few and far between. If the Unico output is really high, then the preamp volume might be too sensitive but it should be able to attenuate the level without difficulty.
If all else fails, you can build an L-Pad with a few resistors. If you don't know how to calculate the values, there are plenty of them on the web. The first one that comes up on a Google search is here:
http://www.lalena.com/audio/calculator/lpad/ This is designed for loudspeakers but it's entirely applicable to attenuating the level before your preamp. Just enter the preamp input impedance in the Z field and something like 6 in the attenuation desired and it will calculate the resistors you need.
mike