Many multimeters have a decibels scale for measuring the power of amplifiers. Often the measurement is relative so if it goes from 10-22 the measurement will be 12. It doesn’t matter what input is used as long as it is within the input range. It is easier to measure the output of an amplifier in decibels than the voltage. Strictly speaking a decibel gain is a power gain not a voltage gain. Power is equal to voltage x current therefore to get the decibel measurement it needs to be squared. A gain of 20dB is a voltage gain of 10 but a power gain of 100.
A word of caution if you connect a multimeter to the mains even if it’s rated higher you probably need to put a load on the circuit even if it’s just a bulb otherwise particularly if it’s an anologue meter it’s liable to cut out. Normally there is a cut out circuit and it just needs resetting. Some modern multimeters don’t connect directly to the circuit and some have a computer interface. The book I read to research this post was Getting the most from your multimeter by RA Penfold which is a very good book I got from Maplins.
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