michael_d,
The controlling device is your (stereo or multichannel) preamp--either a separate one or the preamp section inside your AV receiver.
The level control in the preamp meters out the low varying voltage to its pre-outs, depending on the level setting, then to the amplifier inputs. Where you set the "volume" control adjusts the preamp circuitry to send a signal that will vary (with the dynamics of the music signal) from a few millivolts to perhaps 1 volt or more. As JohnK mentioned, most amplifiers have 28 to 30 dB of gain, which means that if the amp section in your outboard power amp or inside your AV receiver is getting a 1-volt input signal, it will be close to delivering its full output to the speakers.
Alan
