First, thank you for taking the time and effort to describe your experience in some detail.
It seems that the core of your story is the lowered noise level which you say is audible in some way. I have no doubt that lowering the audible noise floor can have a generally positive effect on the sound in many ways. Can you hear the difference in noise level from the speakers when the Ethernet is connected/disconnected from your amp and preamp? Can you hear a difference in noise level from the speakers when you change that critical cable?The first thing I noticed was the noise floor, I use a Naim Nac52 preamp and barely needed to get to 8 o'clock for loud.
OK. If you can hear it, surely it can be recorded with a microphone and measured without any fancy equipment.I have swapped in the old cables, it is not subtle and highly repeatable. Measureable, not sure, though someone on the PSaudio forum has some interesting scope pics. Happy listening.