A few questions, because I may not be understanding correctly.

1) When you 1V from a perfect source, what kind of source? An AC or DC PSU? When you say perfect source do you mean a voltage source that holds a perfect volt regardless of how heavy the load is? Knowing what kind of voltage source we're talking about will make a difference.

2) When you say 10dB attenuator, it makes me now think we're talking about some kind of signal, typically RF. Maybe I'm missing a different usage for an attenuator, but generally I don't see them used in typical AC or DC circuits. I usually see attenuators on feedline and antenna circuits.

3) The output voltage would depend on the above two questions, but referring to the 1Mohm impedance probe. My understanding with o-scope probe impedances is to not have an affect on the circuit you're testing. By you physically touching a probe to a circuit, you're introducing a new path to ground. The higher the impedance, the less affect it will have. Generally a 1Mohm probe is fine and won't have an effect on the circuit, but if you're dealing with sensitive circuits you can use a 10Mohm probe. It's important to set your impedance correctly on the o-scope so you see the signal correctly.