multimeter question

thats not the way i learnt an amp should be set up.

on amp, turn gains all the way down. turn h/u all the way up. back it off if the sound isnt clear, but keep it as high as possible. now start turning amp gains up until you can just hear distortion, then back them off a fraction. all done.

this way, the system can never be turned up too far.

I'm not 100% positive but I don’t believe you can measure the max input voltage (gain) of an amp with a voltage meter. And If you measure the speaker output you will be getting the wattage (watts squared divided by the independence = rms watts). However If you do find the answer please let me know

The short answer to your question is no, you can't measure input voltage requirements with a multimeter.  Input voltage on an amplifier (or anything else) is not something that is externally measurable, it is a function of the amplifier input-stage components.  There's no voltage to measure - this is an input sensitivity rating and not something the amplifier "generates."

That being said, what you are really trying to do is match the input sensitivity of your amp to the max output voltage from your HU.  Some high-quality amps (like JL or Planet or McIntosh) have calibrated input adjustment pots, so if you know (or measure) the max output voltage from your HU preamp, you can then guess-timate a match by setting the input poteniometer to the corresponding setting.  Fine tuning from there can be done by ear or by measuring power output as described in this thread and elsewhere, but getting it perfect requires a tone generator and oscilloscope.