makers should show a few output related specsVery good comments on the output impedance - it is why it is why I always look at the value of the output coupling capacitors. If they are not clearly seen I measure them!
Some tube preamplifiers having less than 500 ohm output impedance will not "distort" with a 5 kohm load, but will show higher distortion with such load than with 50 or 100 kOhm. In my experience most of those I used sounded better with loads higher than 50 kOhm. Unfortunately most preamplifier reviews only show us distortion loaded with 600 Ohm and 100 kOhm.
Voltage se and bal
Imp output at a min of 5 freq at given loads
the size of the cap I think
ralph can better explain what specs by documented testing should be