exponential potentiometer

To Shunt means to direct it to ground. The e-pot is probably an
exponential potentiometer. There are many types of volume controls.

If were connect the + and - of the input on an amp. . .no difference
in signal will reach the amp and thus no output. If we slowly increase
the resistance between the + and - then some signal will reach the
amp and the rest will go back to ground (or short). Finally, if there
is and infinite amount of resistance between + and -, then the amp
will get all the signal.

True. . .the exponential potentiometer is between the + and - and
not in the signal path; the shunt is the potentiometer and is parallel
to the amplifier. . .thus changes the impedance seen by the preamplifier. If you run the pot down to 5 ohms then the preamp
will see just less than 5 ohms. . .not a good situation. . . .

You should have at least some resistance in the signal path to
prevent the impedance from dropping too low. . . you want your
preamp to "see" a reasonable load. It would be like running your
power amplifier into a 1 ohm load. . .it won't like it! It will get hot,
it might shut down, it might stop working. It just cannot supply
enough current. . .it needs a load.

Preamps need a minimum load also. Put a 5K in series then the
shunt pot. . . if the pot is 0 ohms. . .then the preamp "sees" 5k and
the amplifier will be silent since it's input is shorted. . . if the pot
were at 1 million ohms. . .then the preamp will see something just
under the original amplifer impedance plus the 5K. . . .keeps
the preamp loaded.

Let me know how it works.