I have one comment that is a bit rabbit-hole-ish and one that is the opposite.

With regard to the rabbit hole... You could reduce the test kit to just two testers by using trim-pots instead of fixed resistors. That would allow you to test different resistor values for each cap without removing the tester. Once you have it dialed in, you would just use a ohm-meter to see what value you your dialing-in came up with.

Do you hear much difference between the various options?

I also wondered if the cap-resistor-in-series approach might take out a bit of the bass on a humbucker...and whether that might be a good thing, as in the G&L PTB mod...

McCreed's PTB thread:
https://www.buildyourownguitar.com.a...ad.php?t=11557

I think pretty much all passive G&L basses come with this tone circuit, and I find it useful, since the bass pickups (to my ears) benefit from trimming some of the high end and a bit of the boom on the bass end... it made me wonder if the cap-resistor-in-series mod might have some of the same effect as the volume pot gets turned down?

That thought took me back to the rabbit hole: On most of my guitars/basses I play with the tone control somewhere between 10-0, so there is room to turn the treble down or up. It seems to me that this could bring one back out of the rabbit hole.

There is an interaction between the bleed circuit, the volume control and the tone control on the guitar/bass. The tone control is my go-to if I hear too much--or too little--treble. When I turn the volume control down, I tend to turn the tone control up, and visa-versa. It seems to me that the treble bleed would make it so I don't need to do that as much--but I would still have that option.

So when you do your testing, here's what I wonder: How much difference do you hear using the different tone-bleed circuits that you can't adjust for with the tone control?

Nothing beats ears for figuring out what you prefer, but one of the reasons that I like the circuit that you picked is that it seems pretty middle of the road--and I am assuming that you can compensate for slight variations with the tone control.