The amps input impedance should be 10 times the output impedance of the preamp. This is considered the minimum and since the preamps output impedance varies with frequency, it's better for the amp input impedance to be 15 or 20 times that of the preamp.
So if your amp is 10k, then the preamp should absolutely be no more than 1k, and to be safer the preamp should be in the range of 500.
So if your amp is 10k, then the preamp should absolutely be no more than 1k, and to be safer the preamp should be in the range of 500.