Level matching for a/b testing?

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 

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