Looking at the waterfalls in (3 vie con imbottitura...) , it still looks to me like your absorption is only effective down to about 1.6kHz, which is an improvement but isn't low enough to say for sure whether the dip centred on ~750Hz is due to the loudspeaker system or out of phase reflections, assuming nothing else is interfering. Do you have very bare, highly reflective walls and ceiling? Try sweeps at a closer distance, say 50cm, experiment with 30cm also, keeping and possibly thickening your floor absorption. There's also the possibility of aiming the loudspeaker upwards to avoid a bit more floor reflection maybe. The aim is always to avoid and reduce as many reflections as possible and down to as low as possible. Where there are reflections and resonances, you're in guessing and approximation land. For a quick look at what might be happening, try using pink noise while shortening the measuring distance and see if the dip remains. Go in very close to the driver centres and flush with port output, if there is one, to get an idea of the nearfield responses.