Variac noise

@ccogopher A stepdown transformer will be more expensive. Bucking transformers are fairly cheap, since the transformer does not have to be nearly so large.

For example, the Marantz 8 draws 190 Watts. It runs on 100V if the newer Japanese version. That means it draws 1.9Amps at 100V. So a transformer that puts out 20V at 2Amps should do the job. You can get one of those off of ebay, such as this one:

Hammond 20V transformer

The primary of the transformer is simply connected the the AC line to power the transformer. The secondary is put in series with the amplifier. The phase is important; one way you hook it up it will add 20V to the line voltage, which isn't what you want; the other way it will subtract 20V from the line voltage. This little transformer is only $18.00.

If it were me I'd get one with slightly more amperage (3-4A) since I don't like running transformers at or near capacity. But even so such a transformer is going to be inexpensive.

So this is a simple hookup. You could put the transformer in a box so it looks nice. Bud makes boxes that could do the job. Even with the box this is still cheaper than running a Variac or stepdown transformer that has the current capacity to do the job!

FWIW, your variac might be vibrating as much or as little as your friend's. Most of the time when you hear them, you're actually hearing something near them that is vibrating with the variac, rather than the variac itself. If that is so, silencing that bit (for example, a panel) can be inexpensive as well and since you already have the variac, might be a prudent way to solve this.

@ccogopher 

Again, variacs are not meant to be quiet. Your friend is lucky.

You could try a DC blocker on the AC line; clearly you're not interested in the method I outlined earlier, although it would solve the problem as well.

@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...