Do no harm to a signal! Does this mean.....

Name a "bad" wire!

Monster, from what I've seen.  

Realistically if you can’t tell the difference and you lose The test you’re an idiot for spending the money on something that doesn’t make any difference. Correct? Are these test that you’re talking about this simple?

Almost.  @russ69 is right, you're not going to pick up the difference between speaker cables by listening to headphones.  You need them connected to loudspeakers that you can then audition.  

But no audio store would ever do such a test, because too many people walking in there wouldn't know good SQ if it walked up and clanked them on the head with a sledgehammer.  Evidence:  Popularity of Bose Acoustimass systems. 

Does anyone understand what happens to an analogue signal when it flows thru a speaker cable??

Sure.  The resistance, capacitance, and inductance of the speaker cable all have effects on the signal.  

How dramatic those effects are depend on the type of wire (copper v aluminum v silver etc...) the thickness of the wire, the length of the run, the proximity of the two conductors to one another, the quality of the insulation... 

Tons of factors.  Each one changes one value over another.  Some matter more than others.  *There is no perfect solution* is the most important thing to remember.  Just solutions that prioritize one value over another. 

Heavy gauge wire decreases resistance and inductance, but increases capacitance. Light gauge decreases capacitance, but increases inductance and resistance.  

But there's more:

Speaker wire crossing another electrical cable (power, interconnect, etc) has a deleterious effect on the audio signal.  Because now the two electrical signals are interacting with one another's magnetic fields. 

There are so many variables, essentially there's no such thing as the "perfect" cable.  It's all give and take regarding your signal. 

Inductance is a factor of surface area and distance. Wide spaced heavy gauge conductors can be highly inductive. Similarly, small gauge spaced will be low capacitance.

Agree. Didn’t want to overload him with information, though. Was just trying to illustrate the fact that different speaker cables will do different things to the signal based on myriad factors.

For example, capacitance really doesn’t mean a damn thing if your cable run is less than 2m. Raw resistance and inductance are far more important at that point. Conversely, if the run is 50 feet, capacitance becomes a HUGE issue.

Many variables, but the concept of diminishing returns means at some point there are 0 returns. We do not have infinite ability to hear changes. So most of those things "can" make a difference, but most of them unless the design is poor, do not.

Also 100% agree.  Diminishing returns is why one must carefully balance their approach to system improvement between all the various factors.  If you're spending thousands on interconnects and haven't dropped a nickel into room treatments, you're being dumb, because you're leaving low-hanging fruit on the tree in terms of the overall quality of your sonic experience.