axon, These are only assumptions.
I don’t think the lineout and headphone jack are the same signal. Going by the block diagram of the AMS chip, the headphone amp circuit is built into the chip (it does not seem to be an outboard IC with a preamp feed). The lineout (preamp) signal and headphone amp signal come out of the chip independantly. The lineout will be of high impedance (probably 10-20k ohm) to drive high impedance inputs. The amp out will be of low impedance (probably around 32 ohm) to drive low impedance headphones. Sansafix has stated that the lineout should produce the better signal to noise ratio, which is to be expected from a high impedance signal that does not go through another amp gain stage.
Why is my cable getting a stronger signal - good question. Wish I knew! Note that when you connect to the dock, the volume is adjustable to about 25% and then it does not increase. There is probably more there as calv has explained with his DAC explanation, but it is being limited to avoid signal degradation. Sansafix has also stated that it is at .7v rms p-p line level. The cable is no doubt higher than that. When connected to my Klipsch Promedia with the the Klipsch volume all the way down, I still get some sound output. The input is being overdriven. Why? A big guess here! The AMS chip may have an adjustable preamp for the lineout, instead of being fixed. If it is uses op-amp circuitry and there is an open (floating) input, it may produce a higher gain on the op-amp output. So, if there is a connection in the Griffin dock that grounds an input, and my cable does not have it, it may produce more gain and a louder signal. Could also be that there is more to it than just enabling pin7. I hope to find out more when I get a full drawing together of the Griffin Dock circuit.
Are they interchangeable? Hard to say! If you can’t hear much difference between amp out and dock out with your M3 and beyer DT880s, than a mini-RCA or mini-mini is probably fine in the headphone jack. I think what is killing us here is that the headphone out signal in the Fuze is overly good. It is very clean all the way up to full volume.
