Level matching for a/b testing?

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.

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

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. 

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.

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?

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

See what I mean? Instead of learning to listen, becoming a better listener, they have you messing with meters, reading readouts. Also notice the guys giving said advice are the most well-known non-listeners on the site. Confirmation bias on stilts. But I guess if it makes you feel good, that's what it's really all about. 

Use a calibrated mic for your phone and Audio Tools (or iPhone equivalent).

https://amzn.to/3rntWaa

OP
Hoping won’t get ’er done.
Not ALL responses just plain old rude.
Only ~ 92% of ’em.

Try level matching at the source first if available.

oldskoolmark OP

Use your phone with a dB level app in it.
Play a set "reduced level 1khz sine wave" so it’s at the same volume control position setting as where you normally listen at, note the positions for each of the pieces you want to a/b then play music on both at those volume positions.
https://youtu.be/PyD9cMarVJk

Cheers George

Because now at least you are thinking about it, and maybe a little less likely to waste your time with meters and stuff like all the nice but ultimately less helpful people will say.

@millercarbon I hope that the overwhelming majority of your other 8324 posts are less condescending and more instructive than this response. Otherwise, why do you bother?

db meter, or app on your smartphone

https://www.dagogo.com/audio-by-van-alstine-abx-comparator-review-part-1-audio-store-wiring/

^^^^ is a very useful tool... but a little pricey 

Yeah that's the story. I guess starting out we all need whatever extra we can find. Probably play the same track over and over again too I bet. None of which is necessary- and for as long as it is, this alone is saying you are not there yet. Think about it. If you are so easily deceived, it can only be due to not being aware of the volume difference. If you are unaware of the volume difference, then what else are you missing? And you are gonna rely on what, a meter? To make up for what you can't hear? You see how little sense it makes?