The gain need depends on the power amp input sensitivity/impedance etc, most of big amps from the past as Krell needs more than 2V to run properly, upon this market NAD made an 5V output cd player in the 80s.
I would say that nearly every preamplifier should be able to output more than 2 volts rms, and most more than 5 volt rms. With higher amplifier gain, the ic between preamp and amp becomes more and more critical. Conversely, with a lower gain amp, that ic is not so critical. I don't count buffer stages, they are just a waste of a stage imo.
Just what I have experienced though, FRM.
cheers
steve