If aftermarket premium power cords work, why won’t short runs of premium XLR cables work?

I am sad your confidence in ’theory’ precludes all fact.

@elizabeth Its not theory. I've been doing this a long time, and I've seen plenty of cases where one cable sounds better than another, including balanced.

The thing is though that most high end audio products don't support the balanced standard. They might be fully balanced, but fully balanced and supporting the standard are two different things. The standard is there so that cables don't sound different from one another (amongst other things) but supporting the standard is hard so most manufacturers don't do it.

As a result, you have the experiences that you have, because the gear you used didn't support the standard (also known as AES48; Audio Engineering Society File 48).

You can read more about this here:
http://www.atma-sphere.com/Resources/balanced.php

In my experience, different XLR do sound very different. I had Kimber Hero, Cardas Parsec, and finally Kimber KS1116. Each a big jump in price, and a good step up in sound.  

If this was the case, then the associated gear didn't support the balanced standard.

Whatever boxes are on the cable is not really relevant (and FWIW, they don't really serve a purpose either).

If the equipment supports the balanced standard, then you can use extensions all you like. If the equipment does not, the results are less predictable, but in your case no difference is heard so the answer is 'you're good to go' :)

XLR cables are easily extended, if the source supports the balanced standard. If not, its a bit of a crap shoot.
More on this:
http://www.atma-sphere.com/Resources/balanced.php