To achieve an according between measurements and calculations you, the first, have to determine the Power Point for your sub (with relating to LF range, because your sub may have more, than one PP), i.e. to find the coordinates set of PP inside your sub, the second, place your sub the same position at calculations like for measurements and to compare results. To realize the first step, it's strongly recommended to close the aperture of the phase invertor by soft material due to measurements, because the REW simulation method doesn't use such a feature. Only in case you subject to these conditions, hope to use REW correctly. It's important to achieve a match between calculation and measurement results within at least 75%.