From my education and laboratory research, everything affects the sonics in a system. Yes, power is a problem
due to lack of proper filtering stages, varying voltages, including filament voltage in tube equipment etc.
In general terms:
1) The sources, including TT and phono stage, cd players etc are in the small signal arena,
materials, internal and extenal influences, voltages involved including filament voltages of tubes etc.
2) The line level preamplifier are involved in small signals, routing different inputs, vibration,
channel separation (such as 90+ db between channels), separation of inputs (90db or more and
with minimal sonic degradation such as with solid state influence), voltages including filament voltages in tube
components etc.
3) The amplifier is involved with mainly larger level signals, channel separation, electrolytic capacitors, a range from minimal to complex loads called speakers etc.
4) The speaker is involved with electrical to mechanical conversion, room acoustics, a minimal to complex load etc.
Interestingly, just one poor quality part can topple a superb audio system to average or even poor sounding.
So touchy.
I am sure more could be added in terms of specifics.
cheers.
steve