Optical input = much lower volume

[streamy]

I always thought that an optical output was a fixed line level out. Is that not the case?

It depends on what device you are using. E.g. when you use an RPi with a hifiberry digi+ or the like and use the hifiberry OS it has an internal volume control on their software that can be attenuated. So the output volume is much reduced. I experienced exactly this and was very surprised about the low volume until I found out where to increase it. It was on a web interface where then I could raise the volume. Maybe you are experiencing something similar. It was not obvious.