Others will likely have good suggestions I have missed ...Not me! That was a REALLY excellent writeup, Mitch2.
I'll just add a little emphasis to your mention in the first paragraph that power transfer is completely unimportant between preamp and power amp, as well as for any other line-level interfaces. What matters is transferring the signal VOLTAGE accurately.
And although it's not directly relevant, it may be worth mentioning that high preamp output impedance in combination with high cable capacitance can produce a slight upper treble rolloff. That will usually only be a significant effect if the run is very long, and the cable type has high capacitance per unit length, and the preamp output impedance is high.
Best regards,
-- Al