The respective output impedances of a preamp and the input impedances of a power amp will typically be different for tube units vs. solid state units. Further, the input impedance for solid state amps will also differ between single-ended (RCA) connections and balanced connections.
Which type of preamp and amp are you interested in -- tube or solid state -- and do you intend to use single-ended or balanced connections?
Which type of preamp and amp are you interested in -- tube or solid state -- and do you intend to use single-ended or balanced connections?