If Cables Are Not Tone Controls...

Don't expect those who mix cables to agree or understand.

Nonsense. The only way that statement can be valid is if each source and load are electrically identical, which they clearly are not.

It is possible to turn an excellent system into unlistenable by swapping a single cable from the same manufacturer that does not mate well with the driving and source impedances.

Similarly components can be tuned by using different capacitors and resistors in the signal path. The diverse components have the same value but not the same impedance. Ditto cables.

One company who understands tuning is Noble:

"Some of my work involved tuning the power supply. It may come as a surprise to learn that you can change tonality without even touching the signal path, because the signal originates from the power supply. The impedance curve of the N11's power supply is absolutely homogenous from DC to 200kHz, which creates a very balanced sound. I also tuned the resistors for the voltage gain, using a mix of carbon and metal resistors to create a neutral balance. There are a lot of preamps that claim to be 'neutral' or 'in balance,' but there are different shades of 'neutral.' If you have a tube preamp, for example, 'neutral' is at a different level than solid-state; it's not better or worse, but it is different. It took a lot of work to find the tonal balance I like a lot that measures well, with low noise, and fits very well in the Noble Line."

from MBL Noble Line N11 line preamplifier

 separate the normally dual speaker wire into individual ones and place them away from each other

Ummm, maybe not
Capacitance will drop, but Inductance will rise as will Impedance.

Better bet is to buy well engineered cables that integrate well into the system.

I cannot upgrade it because I am near top of the line

cost ≠ sonic quality. Cable properties interact with the devices they connect.

I can't count the number of times an upgrade wasn't with one piece of gear and was near nirvana with another.

In the studio, sometimes the original 1950's mic cable gave the best sound and sometimes the latest and greatest prototype.

Cables are actually so much more than tone controls, that's about the most simplistic description

Cables are meant to transfer electrical energy from one place to another without affecting the signal. They can't. A Tone Control is a device to change frequency response. Cables always change frequency response variant on the connected devices. Ergo, cables ARE Tone Controls.