Regarding the A/B comparisons on sound etc... Just to share, this is what works for me (and a bit of background for context).
Context
I have been into hifi for 35 years (since I was 13 years old and build my first DIY speakers).
I'm an electronic engineer and have worked with design and repair of analogue circuits, both transistor and tube. I no longer work in this field professionally, but I can still understand most of what goes on in a circuit, read measurements etc.
I have been involved in beta testing and voicing of the sound on a number of products, both DIY and commercial products
I'm a mucisian, so I get to hear how instruments and voices actually sound on a daily or at least weekly basis.
How I do when I compare the sound between A and B?
1. Training
It's my experience, that we can train outselves to listen, to remember a sound and to compare how things sound. I can't tell how this actually works in the brain, but it's my experience from listening through the years. I try to have two ''modes'' where one is just listening to the music, enjoying it and not at all listening to how it actually sound. And the other mode is focused more on how it sounds, than on the music. With time I feel one can focus on these modes and listen in these modes.
2. A/B comparisons
For me personally I can't compare sound in these A / B comparisons, where you listen to a short piece of a track, switch over, listen to the same again etc etc. I get confused and I can't focus. For others this works very well...
My process is to listen to a product (or in this case FW version) for a period of time, something like a few weeks. In this period of time I try to be in the ''music mode'' described above, so I just listen and try not to listen for the sound. By doing this over time I get kind of a ''memory'' for that sound without really trying to analyze the sound.
Then I switch to something else, and then I go into the more analytical mode where I really try to focus on the sound. What's my feeling/intuition on this? Better? Worse? Why? What's changed? All this within a short time frame of a few minuts. If I don't grasp it I have missed the window of oppertunity and will start the process over. In this second round I don't need a few weeks, more like a few hours of listening, getting the memory of the sound back again. And then do a new comparison.
3. Listen to real music
My ''calibration'' or ''reference point'' is always how instruments and voices sound live in the real world. I know that our recordings have all been mastered and edited, but for me it's still the best reference to make the comparisons between A and B with real music as the calibration point or reference point. As mentioned I'm a mucisian so I listen a lot and without this I don't think I would be able to get a grasp on the sounds between products, FW's etc. So go and listen to some live music where you can actually hear the instruments. Not a rock concert through a PA system, but more like a small jazz club or something where you can hear the piano, hear the drums, hear the acoustic guitar etc.
This is my process, and what works for me. Just to share experiences. I'm not in any way saying that this is the correct and only way.