Seems like you understood several things wrong about this.

First: The fiio E10K doesnt have a 3,5mm speaker out. It has a 3,5mm amplified headphone out, a 3,5mm line out and coaxial out.

Also you are misconceptioning something about line out and pre out.

line out (Line level output) and pre out (pre amplifier output) are in this case practically the same thing.

A Line out just means an output at line level ( https://en.m.wikipedia.org/wiki/Line_level#Line_out ). This output is analog and not affected by any volume settings (at least if your computers settings are correct).

A pre out is a line level output that is meant to be wired to an amp. Thats it. It is both the same thing. Its just a question of how you wish to call it.

So once again: the fiio E10k olympus has an amplified 3,5mm headphone output that can be set to high- or low-gain to fit different headphones and is affected by the devices hardware volume knob.

On the back there are a coaxial rca output. This is a digital, unprocessed output that bypasses the devices dac completely. (meant to feed another dac)

Also on the back there is a 3,5mm line level output. This is an analog output that is not affected by the devices hardware volume knob and is meant to feed an amplifier.

Furthermore should a dac under windows always be set to be used via WASAPI or ASIO instead of direct audio how windows normally manages it. This way you bypass all windows post processing like reducing the gain or resampling the signal what could harm the signals quality.