I used to think pricey interconnects were snake oil...

Once you have a compatible set of components that sound great together, speaker wire, interconnects and powercords can make a very significant improvement in sound quality. The better your system, in general, the bigger the improvement you are likely to experience. But also, experience developing listening skills will reveal aspects to sound quality you will unlikely have even noticed when you started out.

One truth that has shown itself to me over and over again is that there are lots of good sounding systems, but all the truly great sounding systems have been meticulously tweaked through wires, vibration control, and room treatments in order to get their sound.

While the is no real wrong way. Most of us do components, wires, then room treatments. 

The question becomes what is the incline of diminishing returns for cables?

My own guess is that it's pretty steep, because compared to other elements in the system, cables make a smaller contribution.

Thus, the killer question for anyone spending on cables is: Are cables more deserving of your upgrade dollar than anything else in the system?

For example, if one were to spend $4k on interconnects, the question I'd ask is whether $500 in interconnects and $3500 elsewhere in the system would be a better investment. That's just the form of the question, and everyone would fill in the amounts with whatever suits their situation.

Jasonbourne will buy your old cables.

@hilde45 your perspective is similar to mine - assuming your interconnects and cables are half decent to begin with - the reality is that many are not. Steeply diminishing returns after say $500/m or so.

Now a brief rant about digital cables - these in particular are not worth spending more than a small investment ($300?). I’m a computing expert but not a networking expert but had a hunch that error correction in the protocols (plus the digital binary nature of the information) negates the traditional benefits of better copper and interconnects. One of my good buddies is a networking expert for one of the largest SPs in the USA. He cannot conceive of how say typical electrical signals could change the digital info enough for protocol correction to not work. if this were true, the Internet would not work - voice calls would not work, etc. 

They aren't snake oil, they are just a billion times more expensive than a good tone control.