need help with troubleshooting REW

Greetings, i hope someone can help me out with this one. In the included screenshot and MDAT file, you can see that the measurement seems highpassed with a very steep filter at about 3.5KHz. But i have no filters engaged in the signal path, at least not that i'm aware off. The sweep sounds and feels full range, the house shakes in the start of the sweep. The inputs are selected directly and there are no plugins inserted anywhere in the Apollo mixer (screenshot included). I think most relevant info is visible in the screenshot and MDAT. I can add that the speakers are Neumann KH310's and that in this measurement only the left speaker was switched on. I've checked the mic and speakers with a separate tone generator and RTA. The problem is not present in that scenario: those measurements are full scale, as you can see included screenshot. So, does anyone have an idea of what's going on here? why do i only have a partial measurement?

The problem is solved with the help of John from REW. I'm using the acoustic time reference now and get correct measurements.

excellent! John's REW invention is an amazing bit of work.