Upgrading

@adg101 - Placebo and expectation bias cannot be completely ignored.  I do not have the ultimate system, setup, etc., and I have heard differences in my system from cable changes.  So far it's been limited to analog interconnects (upgraded twice with positive results both times) and some power cords (mixed results) for me, I have not heard a difference with digital cables and I'm almost happy to avoid another rabbit hole.  I have not played much with speaker cables in my current system because I have a splitter as my speakers are shared between my HT and stereo rigs so I've just chosen to stay with what I have.

I did do the classic hardware store wire vs. $2k Nordost speaker cables using a Pathos Logos amplifier and $30k Focal speakers.  There was a difference, but nothing even close to justifying the increased cost.

I also find that I only really appreciate changes in my system after I've spent a lot of time listening to the current setup and then can almost always identify a difference (or not) pretty quickly.  Doing A/B comparisons rely on muscle memory and never seem to be very definitive to me.

@malbor - I'm not sure that your cables would be expected to degrade over time, but I'm sure there's always something bigger and better out there.  When I did my upgrades I started with power cords because the ones of interest were very affordable/cheap and after having a good experience I tried an entry level interconnect.  That experience led me to max out my budget (i.e. what I was willing to spend) on another cable farther up the line which also yielded good results.  I tried a digital cable from the same company and heard nothing, so just let it go.  I did try a more expensive PC on my DAC because I found one used that was of interested and didn't hear anything different that I could definitively point out.

My advice would be to not overthink it.  There's almost infinite options, so you have to be intentional about going down a financial rabbit hole (unless that's what you want to do).

@theaudioamp - I don't think that that it's necessarily expected that the best sound will be the result when using cables from a single manufacturer.  I think it's reasonable to expect that a cable manufacturer would do their R&D with all of their products with the potential benefit of some synergy across their lineup.  Because they are using different components results will vary.  If the manufacturer listens to their products as part of the development process and you believe that cables have a signature sound, it makes sense that you are likely to prefer the same signature sound across your system.

Another way to to say it is that I wouldn't assume that a system with all matching cables will sound better than one that doesn't, but I do think that a listener is likely to reach a pleasing result if they stick with a single company once they determine that they appreciate their signature sound.