Level matching for a/b testing?

MC - what folks are doing is removing the factors that deceive the ears so that when they listen, they know they are listening to actual differences not perceived ones. This means that they can make better decisions for their system. 
They (but not you) appreciate that like any sense, hearing can be deceived. Having you with your saggy old ears and obvious tendency to be subject to deceptions tell them that they ‘just learn to listen’ is unwelcome. This has been pointed out to you hundreds of times. When will you get it?

You play a test tone say 200Hz and check the AC voltage at the amps speaker terminals. Then you adjust volume to match a target voltage.

Using a dB meter is quick and dirty but it is not as accurate as checking voltage.

Level matching would work great but all loudspeakers are not linear. Often loudspeakers sound best once they reach a certain loudness level. So if you are comparing loudspeakers, you want them ideally placed and set at the level they sound best. 

good points raised by russ and mijostyn

i typically use 1 khz test tone to run db meter level check... but of course, different things being a/b-ed have different amplitude responses up and down the frequency spectrum, so with level set equal at 1 khz, you will then hear how they sound differently when hearing actual music played

For electronics up to the speaker terminals, any DMM will do. It doesn't need to be accurate. Just set A and B to the same value.For loudspeakers, a meter or phone again it doesn't need to be accurate.For speakers a mid-band pink noise is the best source.The hard part is keeping the speaker meter relationship the same.