The most accurate way is with a voltmeter across the speaker terminals while test tones are playing. I've also used an SPL meter with pink noise (C weighting, slow response) to get a rough level match. Hopefully you have a volume control with small enough steps to make matching levels possible and a readout to make it easy to switch between different settings.
What is a cost effective way to level match, for comparing different hifi components?
Over the next little while I am planning on trying out some new DACs in my stereo system, and I know that it is important to make sure that sound levels are carefully matched when comparing. So I would like to ask others here how they do this? I realize that these days we all have smartphones and can download apps to measure SPLs. Is this accurate enough for careful comparisons. I wouldn’t mind getting some ideas from others on how they do this, and what might be the best way.