My first thought is the ground connection strength that is somewhere is not tight enough. Thoroughly check the preamp's input and output jacks. Check if it's loose anywhere within the preamp chasis and even amp's and speaker binding posts. Try to swap interconnects and check the plugs on your current ones.
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As you increase the gain of the unit, there is less signal attenuation taking place. As such, any type of signal that your equipment has fed into it is amplified to a higher level.
With all of the above in mind, i would think that you have some sort of a low level ground loop, a poor connection from component to component or a poor connection ( cold solder joint or loose wire ) in one of the components themselves.
In order to track down what it is, i would suggest unplugging EVERYTHING from your amp and then conducting the same experiment. If nothing hums or buzzes, hook up one component at a time and then do the same thing. It could be one component on its' own or a combination of several components with different voltage potentials on the ground. Reading the post that i made entitledNoise, Hum and AC Polarity might help sort out a problem like this. Sean
Thanks Sean and Marakanetz for your input. What really puzzles me though is how come this buzzing sound only makes itself noticed when I physically touch the amp's volume control. As I said in my post, the buzzing sound completely disappears as soon as the physical contact between myself and the amp is severed.
First two lines of Seans comments tell everything for you to know why physically it takes place. You simply decrease resistance(or more precisely impedance) between signal path and ground by touching chasis. To dig onto the precise details of the event you describe requires a basic knowlege of electronics where you should seek out in the college books.