Level matching for a/b testing?

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.