Looking at it again, the target response curve doesn't match the profile of the M5 sound power. Why does ARC roll off the highs so much after 5KHz? Maybe it would sound even better if the roll off was more gradual. If you look at the M5 sound power curve, it starts rolling off at 200Hz by about 5dB per decade.
Then again, your room is not an anechoic chamber so getting the target curve to match the M5 sound power curve isn't right either. Ian has said the in-room curve shouldn't be flat. So what should the target curve look like?