The vari-z is simply a variable input impedance. With the knob fully cranked, it's at 1Meg like any guitar preamp input or many pedals. You can dial the impedance (aka: Z) downward all the way to 33k. But the thing is, the vari-z only really interacts with passive pickup connections. If your guitar is buffered before the Black Box then the vari-z almost does nothing. But if you have a passive guitar then dialing down the vari-Z tames the treble resonant peak in a magnetic pickup making it sound flatter, smoother, warmer, darker. This is very handy on sharp and peaky pickups and especially on steel guitar pickups.
If you connect a passive acoustic pickup to the BB, then the vari-Z does the opposite. Instead of controlling the treble peak as with magnetic pickups, the Z-knob will control the subs and low bass response of a piezo pickup. This is very handy in taking out the unwanted boom and thump in any acoustic instrument like a fiddle, mando, guitar, bass, etc. But if the acoustic instrument is buffered with a battery powered on-board buffer or preamp, then again the vari-Z does virtually nothing.
So for those of us using the Black Box with a buffered signal, the vari-Z knob can be set pretty much anywhere without hearing any change. I typically set it at noon as a nominal load value when used with any buffered guitar. The real benefit is the vacuum tube circuit that gets to work its tubey magic on harsh or glassy transients and to help enrich the midrange as tubes naturally do, harmonically. And the tube naturally helps with that feel thing, the subtle dynamic smoothing.
... and it's just like any other day that's ever been...