@ccogopher Variacs usually make some noise.
There is an alternative way to get the right voltage. You could use a small transformer that puts out 15-20Volts and use it to buck your line voltage down to 100V (or alternatively, from 100V to 120V).
This is done by powering the transformer and then putting the secondary winding in series with the AC line to the preamp. It will boost or cut the voltage depending on the phase of the secondary connection.
The nice thing about doing it this way is the transformer need only be rated about 20% above the current draw of the preamp so its inexpensive.
You might have to have a technician set this up for you if any of this sounds mysterious...