the objective of adding a buffer is for maximum power transfer from the buffer's input to output. hence, if youll be using the buffer inbetween your preamp and power amps, your preamp's capacity to deliver maximum power to you power amps will be improved.
how? the input impedance of a buffer is extremely high as its output impedance is extremely low. if this is not the case, then there will be no purpose of adding a "buffer".