If your sources are balanced, all you need is a switch. If not, feed the switch to a pair of quality transformers to derive the balanced outputs.
You might want to consider this, which is available for about $150 from Zzounds, American Musical Supply, and on special order from B&H Photo Video:
It can accept one balanced xlr input pair and one unbalanced rca input pair, allow you to select between them, and provide the selected pair of signals to xlr outputs.
It also includes passive attenuators for each input, which are claimed to be high quality, but in any event I would think that by setting them to the max position they would be effectively taken out of the signal path. The rest of the device is entirely passive as well, except for a headphone amplifier which can be left unpowered.
I would assume that the unbalanced inputs are not converted to a true balanced pair of signals, as would be accomplished with a transformer. Probably the rca hot is wired to xlr pin 2, and the rca ground (shell) to xlr pin 3. But that would seem like a reasonable way of interfacing an unbalanced output to a balanced input, and as EE professors like to say, albeit in different contexts, there is no such thing as an "ideal transformer."
Hope that helps,