I had the same scenario play out, two of the same model speaker, one used and one pair brand new. Played the same track on both, in the same position in my listening room.

I could not hear a difference, nor could my dealer.

This is proof that burn-in isn’t real.

I measured frequency response output with different power cables and outlets. Made no difference to the very sensitive and calibrated microphone, and was not audible.

I also used a device that measures AC power line noise. The outlet power was noisy, a large and expensive transformer slightly less noisy, and a power conditioner I purchased at a thrift store for $10 was dead quiet. None of this was audible though either.

The cables are bullshit, power conditioning may make a measurable improvement, but you probably won’t hear it.

There are 4, 5, 6 way designs and higher. Drivers usually are designed to cover a decently wide frequency response, so often two or three drivers can handle the audible range from 20Hz to 20kHz.

Things get a lot more complicated when you add more drivers. The crossover, impedance matching, level matching, driver spacing, driver interactions, baffle reflections, cabinet dimensions, time alignment - so many things need to be considered when trying to use multiple drivers in a speaker design.

Kind of making things unnecessarily complex, without being objectively better than a simpler design. Simplicity is the hallmark of good engineering, IMO.

I personally have a 4-way speaker setup, but have heard great systems using single drivers as well.