Hi,
I have several questions before buying the BeePre2:
1) Is it fine for 230V mains or have to be converted / used with a 120/230V transformer?
It has a universal power transformer that can be wired for 230V
2) Balanced input / output is an option. There is a kit for output, but input has to be done without a kit. Correct?
That is correct
3) Any chance the BeePre2 can have a switch to toggle from stereo to mono?
That is something you would have to add on your own. Are you trying to blend the left and right stereo channels in the preamp, or are you trying to get a mono source to feed both channels? Depending on how you want to do this it could be as simple as a switch that combines the left and right input signals. If you want to do it at the output it gets more complex. If you only need a mono setup occasionally you could just use a y-splitter adapter on either your input or output interconnect cables.