Have you tried to plot a graph with both speakers + sub? The sub might help smooth out some of the low frequencies.

I do mine with both mains + sub using the Rives Audio Test CD 2 (corrected for RS meter). All frequencies from 20Hz-10Khz are within +/- 6db at the main listening position. I do have some diy room treatments plus lots of plushy stuff in the room.

BTW, the RS meter is not designed for frequencies above 10KHz.

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