As a person with some EE education and years of video/audio technical experience, measurements that small lead me to ask:

Did you measure with a 'scope or a meter? If a 'scope, what did the display show?

Sheilded leads?

What voltage did you get from the leads when the leads are not connected to anything?

Were the fractions of a mV you reported AC, DC or a combined measurement?

Were the inputs to the 7100 open, with an appropriate terminating resistance, or shorted?

Were the speaker wires disconnected from the amp and the measured output terminated by something like a 10-ohm resistor?

I ask all these questions because there are reasons one might obtain readings in the 0.5mV range even if everything inside the 7100 case were ideally perfect.

I'm not trying to be a troublemaker!