..tell me why I should definitely know exactly why the sound upgrade happens? I am looking for the good performance of my system, and then I want to enjoy my music! Is it about changing the power supply, or the correct support of the audio machines, or the correct placement of the speakers in the space, or the care of my dedicated room?
I knew from the beginning that my views would provoke such reactions. The politeness in the answers, however, must not be lost.
That is what I believe..
I have been a hifi enthusiast for just over 50 years. I am an engineer so I do tend to try to understand why and how things work.
There are lots of things which are said to make a difference. Many of these have sound technical reasons why they do indeed make a difference but, disappointingly IMO, there are several which demonstrably can not be making a difference which are strongly marketed and sell because of expectation bias and the fact that humans have a short aural memory, so are easily persuaded to hear things which are not there.
I like to understand why things make a difference and experiment to get an idea of the magnitude of any effect.
Thes things make a big difference IME.
The listening room.
Particularly the position of the loudspeaker in the room.
Cartridge (for LP playback)
Record player and particularly where it is in the room.
These things make a small difference IME
Protection of sensitive to microphonics valve equipment from vibration.
Power amplifier - is it powerful enough? does it have an output impedance big enough to effect the frequency response of the spoeaker.
These make no difference IME, as long as correctly engineered for purpose.
Interconnects and speaker cables.
DACs, I have compared several level matched and without knowing which is playing - all the differences I thought were there vanished... They were from £1000 to £14,000, including dCS, Linn, Chord etc., I haven't auditioned an inexpensive one.
Power supplies. I have only made one comparison but it was a much hyped improvement which did not exist when I did not know whether it or the standard one was connected.
Expectation bias and the placebo effect are strong.
In the end, the only thing connecting your DAC to the rest of your system is its output. We can measure the output, it only has voltage magnitude, frequency and phase which we can certainly measure to a level of accuracy way beyond the tested acuity of young ears.
If there is no change to the output of the DAC when changing power supply there can not be any change to the sound, obviously, so if you believe you are hearing one you are decieving yourself.
In any case, if somebody who worked for me designed something to be connected to mains power and it needed an accessory supplied by somebody else to work correctly I would consider the employee incompetant and he would be looking for another job. Any DAC which would be improved by changing PS would be a bad design, and the Chord is very good.
it is all to easy to claim these things but when asked for explanations, these people generally fall back on something like "just use your ears!!". 
