I was looking at the 70V speaker systems as while as ESL speakers, I apologise for any mistakes I made. I always thought that DF was the one that had to do with FR changes more so than just low Zout(since line outs ain't that low impedance either) but its just about impedance swings relative to the Zout is that right?
DF is a marketing creation, just like PMPO (okay, not *that* bad, but you get the idea).
FR changes are the result of the relationship between Zsource and Zload - line outs are higher impedance but consider that line inputs are also higher impedance. There's still a potential for mismatch depending on what you hook up. DF is some formula to make Zsource look pretty, iirc it's Zsource/Zload (or Zload/Zsource, whatever makes the bigger number basically) - it's completely arbitrary. It's easier to market things with a "single linear variable" than complex specs; which looks better:
Amp XYZ with output impedance 1 ohm.
Amp ABC with a DF of 5000.
XYZ can actually express a "DF of 5000" into a high impedance load, and ABC might have an output impedance even higher than 1 ohm because that's a unitless number. Make sense?
I meant "R" and "Z" are different.