My friend and I each use multitrack recorders, both of which have inputs that can be given a Hi-Z charge, apparently for high-output instruments such as bass and keyboards.

However, we have done a comparison test in which we plugged a passive bass (direct from the instrument on one occasion, and direct from the bass amp on another occasion), a passive guitar, and a Yamaha Portatone keyboard into these inputs and we listened back to recordings we did of each instrument first with Hi-Z on and then with Hi-Z off.
The only perceivable difference was a slight rise in volume when NOT using the Hi-Z charge.

This left us with the question: how and when should the Hi-Z be used exactly?
the Hi-Z refers to High Impedance, not levels or "Charge".

guitar pickups are high-impedance, most mics are low-impedance.
Hi-Z's supposed to be for guitars and bass plugged directly for a clean sound. If it isn't a big deal in sound though, go with what works for you.
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Recording Guitar Amps 101