The simpliest way is to take preamp of the same brand as amp.
Other cases you should match the input sencitivity of amplifier to the gain of preamp:
If input sencitivity of amplifier is low than preamp should have a high gain or the other way arround...
Not all cases are trivial and you should also know the other variable such as input impedance. In trivial case the input impedance of amplifier should be 10x of output impedance of preamp but this ratio is valid if the output of preamp is relatively high since transfer of voltage has substantial losses and the losses are certainly not linear since you're dealing with complexed signal i.e. music.
The ideal case is when input amp's impedance is too high and the output impedance is too low but if the output voltage capabilities are substantially higher than input sencitivity of amplifier than even 10x-rule has its own tolerances in that case...
Other cases you should match the input sencitivity of amplifier to the gain of preamp:
If input sencitivity of amplifier is low than preamp should have a high gain or the other way arround...
Not all cases are trivial and you should also know the other variable such as input impedance. In trivial case the input impedance of amplifier should be 10x of output impedance of preamp but this ratio is valid if the output of preamp is relatively high since transfer of voltage has substantial losses and the losses are certainly not linear since you're dealing with complexed signal i.e. music.
The ideal case is when input amp's impedance is too high and the output impedance is too low but if the output voltage capabilities are substantially higher than input sencitivity of amplifier than even 10x-rule has its own tolerances in that case...