Level matching for a/b testing?

Two ways.  One for digital and another for analog for me.  For digital I play a Song at the level I like on the original equipment.  I then play a test tone with my test cd, i measure the level of the test tone,  I the swap the piece i’m comparing. I play the same test tone and again measure the level and set it at exactly the level as before.  I make a note of where the levels were set on the pre-amp for each piece and then A/B listen,  

the key is knowing where on the Pre-amp to set so the levels are the same.  The only way to really get that right is with test tones.  

People can and off times do misinterpret level/volume differences in equipment as being better or worse,  or, if things are heard at different levels one may hear more detail on one piece and think that piece is better when it was just turned up louder,  apples to apples, make sure the levels are matched first. 

Same test for analog, just use a test tone from a record and measure  and match the levels. 
Enjoy
Enjoy

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