spl meter would be ideal,, but don’t have one and not that serious about it.
You can always use your cell phone/tablet along wiht an app like Audio Tool. Even without a calibrated microphone you can precisely match levels.
Roon vs Lumin App
I extensively tested Roon vs Lumin app and my findings for my system is no difference or splitting hairs for any noticeable difference if any. When I began the testing it seemed obvious that the Lumin app was sounding better, but soon realized that the volume output is significantly greater from the Lumin app which obviously makes it seem fuller sounding. I was able to balance the output as best I could by using the output meters on my amp playing the same song matching as close as possible. To make matters worse the variance is non linear, can vary with different music, and can vary as the associated equipment warms up, etc.. Key is pay attention to the output to equalize volume for an accurate comparison. I have my Lumin S1 direct to MC452 amp optimizing leedh volume in roon and the app.