A proud VHT 2502 owner here

There's something that's been bothering me for quite some time...
How does one ensure the correct level is coming to an amp from a preamp?
I've got several preamps, each with a different output level selector switch and, obviously, an additional output level select knob. I know the difference between signal level categories like Mic, Instrument, Line Amateur and Line Pro but I have no idea how to actually measure the output of a given preamp in dB.

There's also the impedance thing - Each preamp has it's own output impedance and I can't find information on the input impedance of the 2502 anywhere on the web. Does someone know how much it is and effectively what a load it is seen as by a given preamp?