BUT - No signal attached, some amplifiers will hiss when wide open. Turning the pot down it goes away. Can't blame the hiss coming from the input signal if it's not there to start with.
How does that same amp behave with input sorted? At the risk of getting all "circuit design" with you, a typical input attenuator is just a potentiometer with the pot wiper feeding the amp input circuit. If the amp is using a bipolar device in its input LTP (long tail pair differential amp), that input will have a noise current associated with the input.
When the pot is full up, with no input, that amp input device will see roughly 10k ohms to ground. The input noise current will be converted to noise voltage by that current times 10K. When the attenuator wiper is all the way down, the resistance seen by the amp input will be more like 0 ohms. Alternately if the amp input was shorted, or connected to a low impedance source, when attenuator was full up it would see a lower impedance (for less noise). In fact with input shorted you might hear the noise rise until the attenuator reaches -6dB then fall again up to 0 dB. At -6dB the source impedance of the simple 10k attenuator pot will be a maximum of 2.5k or -12dB.
In practice this small amount of input noise will be swamped out by the noise floor of the stage feeding the amp.
Back when I was dealing with customers I would discourage WFO listening tests,,,,
JR
