Input impedance on DAC4800A and all versions of Cherry is about 10K.
As far as a tutorial on impedance matching, see here:http://en.wikipedia.org/wiki/Impedance_matching
However, matching a preamp to an amp is rarely done these days! Impedance matching has to do with power transfer, and the idea is to have near zero power transferred in the first place. For example a 10Vrms sine wave --- a huge signal as far as preamp outputs go --- dissipates only 10mW into 10K.
The real concern with modern equipment is signal loss. Typically, the lower output impedance of the preamp the better, and the higher on the amp the better. The problem with high input impedance (on the amp) is noise --- higher input impedance results in more noise from the input stage, then the noise is amplified!
An amp:preamp impedance ratio of 100:1 or larger is a good rule of thumb. Some say 10:1 is fine, but that results in almost 1dB of signal loss. 1:100 results in less than 0.1dB loss.
Also, we are talking about impedance magnitude here, but impedance is really complex (as in a vector, not meaning “complicated”). Cable capacitance and length (within reason) become negligible factors with audio range frequencies and practical impedances. Here’s a good Wiki on impedance in general:http://en.wikipedia.org/wiki/Electrical_impedance
The only time we have seen the input impedance of our amps com into play is with passive attenuators, where you need to know what load you’ll be driving. Decent active preamps should have no problem driving loads well below 1K. Thanks for reading.