the line out could/should use standard values, but like anything else, between the incompetent designs, the false line out that is just the HO at max volume, and the clever little guys who thought that if they made the line out a little louder, most people would believe the sound to be superior in listening tests without matched volumes, we end up with pretty much whatever.
most of the times the max voltage will be around 1 or 2volt, and the impedance output should be around 1 or 2 hundred ohm(but can really be anything). so it can clearly affect the gain and overall loudness, but it can also make an amp clip. and there is no perfect choice as a low signal might have a higher noise floor, and hiss, or make your amp unable to get some headphones loud enough.
usually line outs on portable devices have lower voltage than desktop product line outs(ipod effect). so portable amps might tend to be designed for lower voltages and clip sooner than desktop amps in that particular situation.
the hm901 LO being louder might just be that it's the only one with a 2v LO while others are lower. I don't know, but sony certainly didn't output 2V into the LO of any walkmans I've owned. the fiio X3 had the loudest line out for me and it's given at 1.7v.
So basically there is no currently portable player on the market that has true line out and use standard values (any output less than 2v automatically indicated false line-out). If the false line out is just the HO at max volume. wouldn't it be better to just use the headphone jack cos then that way, the volume can be adjust digitally within the player itself?
How did you get the value of the max voltage and impedance output of 1 or 2 volts?
If the player with a lower signal has a higher change of causing higher noise floor, hiss or inability to driver headphone to louder volume. then what's the disadvantage of having a higher voltage say 2v?
Talking about 2v, I'm confused as to why Retrias found there to be high distortion from the hm901 line-out.
You trying all sony walkman players and none output 2v into the LO.
What do you think of the sony hd5 network walkman?
There is a LO feature that can be activated in the settings allowing the LO to be used with the headphone jack.