W/o DBTs, the only standard is that there is no standard.
This. DBTs are overly complicated for a reason...you have to completely remove the placebo effect. I don't believe anyone could tell the difference blind between a 30$ DIY power cable (meaning, using good parts) and one that costs a thousand dollars in a blind test. Does this mean there's no difference? Not necessarily, but again, it will tell you if the 'upgrade' is worthwhile from a cash standpoint. FWIW, I don't know for sure if there's a $1,000 power cable, I'm just assuming there is

99% of us have budgets of some sort yet many subjective comments lead us to believe that certain tweaks or minor upgrades are a worthwhile investment over upgrading our speakers.
Consider this, if you were to sell (or just not purchase in the first place)all of your electronic equipment and speaker cables, how much would you have? What speaker upgrade would then be available to you using that plus the money your mains cost? If you were to use those upgraded speakers with say, a high quality cheap receiver (using cheap gainclones or something of that nature if you need extra power), would you have a better experience? In my own experience, I was able to do this and I would never go back. Then again, my mains are 95 db speakers. This is one reason why I believe sensitivity is so important. I include subs in with speakers btw.
There does come a point where diminishing returns kicks in, obviously, but here on AC, most are likely below that point IMO (high-value offerings are sold from the manufacturers who participate).
Dave, I pointed out fairly obvious limitations on single driver designs that are somewhat well known. A big driver hasn't been developed, to my knowledge, that does highs well. If you stick a small driver in a huge cabinet, you have to deal with issues caused by that. Dynamically, a single driver can't do what other designs can. They could sound great, I'm not saying they couldn't...I just believe there are limitations. A perfect single driver is the ideal, there just haven't been any developed yet IMHO. Every design has limitations or trade-offs...hence why there are so many different options on the market. I'd go a different route, that's all. Nothing wrong with liking single driver speakers...I've been meaning to add a good single driver speaker to my collection for years. I use waveguide-based speakers...critique those all you like.
I don't believe all speaker types have the potential to perform at a world class level. Many are going to be restricted dynamically, some are restricted by thermal compression, and all are waiting on technological advances. If electrostats could go down flat to 10hz without a sub, maybe we'd all be using those, but that doesn't exist. If waveguides could provide directional control down to 500 hz, awesome! That would take an enormous waveguide, though...one that is impractical for most of the market.