Typical HiFi preamplifiers have rated outputs (SE) of 1-1.5V for a 150mV input. Obviously, if you put in 2.0V you will swing some very high voltages out of the good ones. I've had preamps that would swing up to 35V prior to clipping and most in my collection will easily swing 10-15V, all with extremely low output impedances. My Accuphase is rated at 1 ohm on the SE (RCA) jacks and 600ohms (correct) on the balanced (300+300) and it will happily swing 12V RMS out the RCA jacks. But the volume pot is around a 3pm to do that.
The issue is "typical" domestic preamplifiers would be a poor match in terms of volume pot position/range with a 12.4dB gain power amp. The buffer IS required to bring these amplifiers to a more typical overall gain. It's not rocket science.