Hey there Mike;
The Volume Pot should be one with an "Audio" Taper - a Logarithmic increase or decrease in Resistance value, so go with the type "B" Taper Potentiometer.
If the type "A" Linear Taper Pot was used, the perceived increase, or decrease in sound level will not be "Linear" while the Pot handle is rotated.
A Linear Taper Pot would result in a gradual perceived increase in Volume Level for the first 40% to 45% of the rotation, then appear to quadruple the Volume between 45% and 60% rotation.
At the end, it would appear to double the Volume for the final 40% (from around 65% to wide open 100%).
The Linear Taper Pot could be used on the Preamp (or at the Output of the Bass Guitar) if you cannot find an Audio (Logarithm) Taper Pot available with the Resistance & Power Values you need.
Probably not an issue finding an Audio Taper Pot, with a Value of 250K or 500K Ohm, 1/2 to 1 Watt rated, and a decent Tolerance (5% and less).
-- Scott
