No need to measure: balanced connections will double the input impedance of your component. This is the drawback to using balanced connections (i.e. differential). You get double the noise cancellation but double the input impedance. The stereo works as a system and what is delivered to your speakers is the system transfer function (input/output). The impact of the input/output impedances on this transfer function are in general small in audio applications. Ideally, you will have a product of the preamp TF and the amp TF (e.g. infinite Zin and 0 Zout), no more and no less. However in the typical case, there is some mismatch and so you get less than ideal - this is the nature of transistor output stages. FET amplifier inputs alleviate this greatly with their high Zin but the problem will forever exist. Capacitance matching is more an issue than resistance actually but this problem is often resolved with good interconnects. Modern design is component-oriented which means the amp is designed by itself and the preamp is also. This is fine as long as you can make them work together independantly. The way you do this is to make Zin infinite and Zout zero. If you are more confused now, email me and I will explain in greater detail.
