I'm not an RF engineer (though I have a MSEE). RF people do everthing
in power, while baseband people think in terms of voltage. In the case
of using an ADC, of course you care about voltage so you don't clip.

I'm going to set up my large Wellbrook on Saturday and plug it in
directly to an oscilloscope with the 50 ohm terminator set. I'll let
you know what I get.

BTW, RF engineers use power meters a great deal. I have a HP 432B which
I'm sure will be able to get a reading from the Wellbrook. However,
unless some signal dominates, you really won't know the proper peak to
peak level due to an uncertain crest factor. Hopefully something will
show up on the scope. The power meter is broadband. I don't know the
lower limit, but the upper limit is 10Ghz.