Thank you. That's pretty much, truth. Asking before buying poweramp for my fronts and the one which I'd like, has 10kOhm input impedance. Looks like it's not a good choice for pairing with Denon.
A few thoughts and some data:
For the Denon AVR's you'll need to go to the X8500H or X6700H, that generally have around 220 ohm preamp output impedances, to increase the input/output impedance ratio to >10.
Denon AVR's with relatively high preamp output impedances have been used to drive power amplifiers for a long time. That doesn't mean a lower output impedance wouldn't be better, but as you'll see below it would likely make little, and more likely, no difference. .
The Denon preamp output impedance should be constant with frequency since it is set by resistors as shown in Amir's data. If the input impedance of the power amplifier is also constant then the only effect is some loss in effective voltage input to the power amplifier due to the voltage drop across the preamp output resistors. For example with easy numbers:
1k ohm output impedance, 9k ohm input impedance at 1kHz. 1V output from the preamp means .1V across the output resistors and .9V to the input on the power amplifier. This is an output drop of about .9dB. If the impedances don't change with frequency, then this is just a slight drop in output that won't be missed since the frequency response isn't affected.
If the input impedance of the power amplifier drops to 7k ohm at 20kHz then the output would drop by 1.2dB at 20kHz. This means that the overall frequency response would drop by .3dB from 1kHz to 20kHz. This is a very small drop. These small drops are likely the reason why relatively high output impedances from preamps are still acceptable to most AVR owners. Loudspeakers will vary a lot more in frequency response.
X4700H and Your Proposed Power Amplifier
In the case of the X4700H and your potential power amplifier the two impedances are 1.2k ohm and 10k ohm. This means the output will be 1.0dB less than if the X4700H had a 0 ohm output impedance. Unless the input impedance of your power amplifier changes significantly with frequency, this change in output level, but not frequency response, won't be noticed.
To make up for the 1.0dB down, the volume control could just be turned up 1dB if listening to pure direct in stereo. For an Audyssey calibration that might be -6dB Left, -5dB Center, -6dB Right, etc., with a really low output impedance, Audyssey would just increase the settings to -5dB R, -4dB C, -5dB R, etc., to account for the -1dB.
If you get an X6700H with an output impedance of 220 ohm, the drop will be about .2dB.
It is very easy to design an RCA input with an input impedance of 40k ohms or more. Unfortunately many power amplifiers have much lower input impedances. This low impedance situation is typical where the power amplifier also has XLR inputs and the manufacturer saves a few cents by using part of the XLR input instead of implementing a separate RCA input.
The above examples are the case where the preamp section is disconnected from the internal power amplifier channel. If the preamp is connected to the internal power amplifier then the effective input impedance the preamp sees will drop. This drop is because the input impedance "seen" by the preamp output will be the input impedance of the internal power amplifier channel in parallel with the input impedance of the external power amplifier.
Using 30k ohm as a nominal input impedance of an internal power amplifier channel, 30k ohms in parallel with the 10k ohm impedance of your potential power amplifier equals an effective input impendence of 7.5k ohms. This will in turn reduce the input to the external power amplifier by 1.3dB. Still not much to worry about unless the impedances change significantly with frequency.
