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.