Impedance matching with unknown filter
Filters are designed to work only with a given source impedance and a given load impedance. The most common cases are :  


in audio frequencies : source impedance = 0 Ohms (&lt;10 Ohms for example), load impedance = Infinity (> 10KOhms for example) 
in radio frequencies (> 10Mhz for example) : 50 Ohms at both ends  


In general, these impedances doesn't need to be very precise (say for radio frequencies : 10% error should not be a problem in most cases).  

In your case, if you don't know for which impedances the filter has been designed, and furthermore, if you don't have an idea of the transfert function you should obtain, the problem can become arbitrarily difficult.  

Hope your filter is somehow classical (50 Ohms, Tchebychev &amp; Co)