I've experienced this myself. When I finally got the right sized (larger) subs for my space, I at first thought something was wrong because they did seem quieter after calibration. I really think it's because the subs I had before were struggling to fill the room so they were distorting. That added noise, but subtly enough that I didn't hear it outright as distortion. It was enough to dirty things up and add noise tho, and when it was gone I perceived it as being quieter. What it really is, is clean bass and it had the effect of cleaning up the entire performance.