Hi!
1) it doesn't change the output-impedance - that stays the same (otherwise a high impedance headphone would change it too -
) but ...
2) ...it increases the input-impedance of the headphones you connect
3) it reduces hiss and noise level should there be any
4) if you connect it without a headphone connected or with one connected on my LGV10 it triggers high impedance mode - the highest amp mode possible
5) of course that comes at a price - it reduces the volume but that is compensated by the stronger amp
The adapter does not change the output impedance but the input impedance (like a headphone, the resistance) so there is no deterioration of sound or frequency response in the headphones. It does quieten the sound as mentioned before. However the better sound quality of the higher amp mode does compensate for that easily.
In the V10 thread was a lot of debate - I think we settled that the line out (ext audio) and high impedance mode trigger the extra amp of the V10 to take over and its sound quality is better.
You can choose to take a 20 Ohm adapter - then you have less volume drop (and still an overall volume improvement if high mode is triggered) and depending on the headphones you use you might still trigger high impedance mode (if headphone +20 Ohm adapter = >50 Ohm).
I hope the above makes any sense...
Cheers!
EDIT:
Example: I just had the RHA T10 connected - with adapter I listen to volume level 45 - without adapter I get the same volume around 40 so not a huge difference with a 75 Ohm adapter.