In my experience, it’s the power cable.
I’ve had expensive cables ever since 1986. Started out with MIT, then when I became an audio writer, changed to Transparent. Also had XLO.
Dropped out of audio for a while and came back in 2003. Saw a new name in the power cord/speaker cable field, Shunyata and was curious. Read a few reviews and thought, 'this sounds like something I'd like,' so I bought the speaker cable (Andromeda Constellations Series) and then the Aries interconnects, as well as Nordost Quattro Fils. Everything sounded nice, but not quite...what I was used to. (Turned out to be the Marsh amp. Too ’polite.’)
Then I saw that Shunyata had power cords, read up on them and bought the Black Mamba (2003 version), Python and then The King Cobra V2 (2003 vintage, their first line of power cords). When I inserted the King Cobra into the system and placed it on my ASL Hurricanes, the system came alive in a rather spectacular way. Dynamic as hell, extremely strong mid-bass, imaged like nobody’s business. So, I’d say the power cords - with a caveat.
If you insert a speaker cable that is a better model than the power cords you have - and they are from the same manufacturer - the speaker cable will appear to be the most important. So, for example if you have Nordost Purple Flare power cords and have been using Red Dawn speaker cable - and then insert Frey 2 speaker cable, it will appear that the speaker cable was the big improvement. But lets say that you then (keeping the Frey 2 speaker cable in your system) then purchase a Nordost Try2 or Valhalla2 power cord, you might think the power cord suddenly "out-powered" the Frey 2 as the bigger improvement.
That said, I’ve found that all items in one’s system need to be equally good, but the power cord, given that it filter out impurities in the electricity in your system, will push the system higher.
I have found, over the years, that, in some manufacturer’s lines, one component compensates for the other. In the old Goldmund components, circa 1986, the Goldmund front end components ( The Goldmund Studio, for example) were just slightly dark (yin), but then, when they introduced a line of electronics (such as the Goldmund Mimesis 9, which I owned), the electronics were beautifully clear and transparent - and it was like you had inserted a 150 watt bulb in a lamp after living with a 75 watt bulb for so long, to the point of the sound being very close to Spectral electronics (slightly "yang" - sounding) speaking strictly of tonality here. But Put the turntable together with the amp (I was using a Convergent preamp, also of 1988 vintage, which is itself slightly "yang") and it was a match made in heaven. However, if you used the Goldmund electronics with other manufacturer’s components, you might well wonder what the fuss was about.
