If I try to set the multiplex frequency, the object doesn't work at all because _nmuxes contains 0 instead of the given parameter nmuxes. Calling one constructor from the other doesn't seem to work as expected.
Others gave me this hints:
You create a nameless local object of type uMuxOutputLib in your constructor
uMuxOutputLib, which is then immediately destroyed again. This local
Object has nothing to do with the first. So the parameters all end up
in Nirvana.
Please try member initialization and delegating constructor. This is how you do it from C++11.