James Steele wrote: ...I'd have to do the ENTIRE RIG to be certain if there's a difference. Oh yeah, and once it's done, it's going to be difficult to A/B against the old right. I suppose I could try record before and after without changing mic placement, but being in the room with it is really the best way to hear any nuances like we're talking about. Microphones, as good a they are these days, won't pick up certain qualities that you can detect being right in the room with the sound source. Also, much of it as far as guitar is "tactile"... that is, odd as it may sound, the difference may require playing the guitar to hear in terms of whether the instrument "jumps out at you" a little more or less...
Being a guitarist myself I completely agree with this, James. Even if you didn't rewire the entire rack and just ran whatever cable you were testing from your guitar into a class-A tube amp and recorded it into
any rig the difference wouldn't be as great as if you were right there in the room with it, experiencing it first hand.
I never trust other people's recorded examples of what different cables sound like. The only difference that can be accurately presented in this manner is the noise floor, and even then I'm skeptical (was the cable running next to the amp's power source, or a rack's power source?). The reason I don't trust these pre-recorded tests is that it is so easy to "accidentally" move the mic a half of a cm towards the speaker or source, thereby adding some "warmth" (proximity effect) and making cable x that much better than cable b.
I do acknowledge that some cables are higher in quality than others, but when it goes into the whole bandwidth discussion of which cables are brighter, or more mellow, or whatever, I personally believe that those added benefits of said [instrument] cable mean nothing in the grand scheme of the entire signal chain (most importantly, with the introduction of a microphone, preamp and ADC). I'll spend much more time moving a mic around on a cab speaker (or speakers) than selecting an instrument cable.
Time is better spent practicing than debating cables, as that makes the biggest difference of all... Ummm, I'm gonna go now...

"We are all disgusting, doomed to our dirty little tasks. Eating and farting and scratching and smiling and celebrating holidays." -Bukowski