Where do cables upgrades have the most impact?

My experience is as follows:

1) I/Cs and digital source power cables

2) speaker wires

3) power cables

Mishan 12-3-2018
Using the example of intercon cables (the same manufacturer and the same model), I can say that in most cases a cable 1 or 0.8 meters long sounded better than half a meter long. I suspect that the matter is in some "wave" nuances, but I would be happy to know the expert’s opinion.

I assume you are referring to line-level analog interconnects.

Most cable parameters are directly proportional to length, including resistance, inductance, capacitance, the resistance rise that occurs at high frequencies due to skin effect, the effects of the resistance of the shield or other return conductor on the susceptibility of the connected components to ground loop issues, the effects of dielectric absorption, propagation delay, etc. (In mentioning these parameters, btw, I am putting aside the question of whether or not any of them may be great enough in degree to be audibly significant in a 1 meter analog interconnect).

The only parameters that occur to me that are not directly proportional to length are what is called "characteristic impedance," and susceptibility to RFI (radio frequency interference) at some frequencies. Neither of those seems likely to be relevant to the sonic effects of a 1 meter interconnect conducting analog audio signals, however, assuming the system is not in an environment having unusually high levels of RF.

Therefore it seems to me to be a good bet that your 1 meter interconnect has more effect on the sound than your shorter interconnect, and that increased effect is simply more complementary to the sonics of the rest of your system and/or your subjective preferences.

Putting it all another way, the "shorter is better" philosophy assumes that the goal is for the cable to have as little effect on the sound as possible, at least in the case of a line-level analog interconnect.

It could very well be a different story in the case of a digital interconnect cable, though, as explained in this paper. And perhaps also in the case of a phono cable, depending on the particular cartridge and phono stage, due to a number of effects that can result from the interaction of the capacitance of the cable and the inductance of the cartridge.

Regards,
-- Al

The one problem with "Shorter is better" is down the road, when you move stuff around, or change racks, or buy a different gizmo. Suddenly your once just right interconnect is discovered to be three inches too short??? (the old box RCA were on the left, now they are far over on the right...)  I long ago used to believe in the just long enough IC. And over the years I have gone the total opposite. NOW, all my newer IC are 1.5 meters long. Long enough I am not going to have to worry about 'TOO SHORT $&@!!$(# !" anymore. And those are $1,100 a pop IC.                           
Just a shout out for buying a definitely longer than just enough when you buy big money cabling. For good reason, a little LONGER is, in the long run (pun intended), better. (primarily for your wallet, and I being a spendthrift cheapskate, watch that carefully)     
On the other hand, if you buy cheap cables, it hardly matters, Or if you change cabling like some folks change clothes. no problem.

Is it really the cable industry that needs our excess money the most?

The ’industry’ exists because someone who BUYS cables wanted something better. One ’better’ cable at a time... It grew. If no audiophile bought them, the industry would not exist. One actual plus for consumers is cables generally do not wear out. I still have cables that I made in the 1980’s. So aside from wanting better... no need to ever buy more. Cable company advertising is only to lure you away from SOME OTHER CABLE COMPANY. No one walking down the street accidentally saw a cable company advert and decided Gee I must NEED those bad, and rushed to buy some.
I am probably like a lot of audiophiles in having a pile of cables I do not use anymore. All told between IC, PC, speaker plus video or digital I must own thirty cables not in use, sitting in the closet. (anyone need a Monster S-VHS cable??? I only keep one because.. never know I might suddenly want to use it.. for????)   
As for "the most".. Priorities. Sometimes you might be best off spending your money on some other audio doodad. But sooner or later you might, maybe, decide to spend that $3000 sitting around on a cable? Who knows.. I did. (and glad I did)