You are making one glaring mistake, you don't set the gains using a DMM with the speakers connected. Impedance is not a static variable it can increase or decrease, depending on the frequency that is being reproduced. Using the DMM method you are setting the amp to produce a target AC Voltage for the nominal impedance of the driver but with no load on the amp. By setting the gain with no load you have a built in safety margin for times when the impedance decreases. It's a safe method as long as the amplifier will do rated, but you will have a little less output.