Heh well I'm in the UK so that room doesn't sound small

55cm to front of speaker shouldn't be bad - the manual recommends to "Avoid positioning the loudspeaker at a distance (dwall) of 0.8 to 1.75 m from the wall behind the loudspeaker.". The SBIR effect with a distance of 55cm would give a dip at 156Hz rather than 70-90Hz where your dip is deepest, so I expect this is not the issue. You might be getting a bit from the side/angled walls if they're about 95cm-1.2m away.
Whatever the exact cause - whether cancellation from a nearby wall, ceiling floor, or a room mode - it may well help to try some other positions, as the -9dB dip will be from the room, not the speaker.
For quicker testing you can use REW to make measurements: you can download your MA 1 mic's calibration file from Neumann and then in REW, use the mic and the calibration file to do a quick bass measurement to check that dip. It could be relieved by a small shift.
Or, using listener/source symmetry, you could place a speaker in your listening position and measure/listen around where you want the speaker for a good spot.
After finding a reasonably dip-free spot for bass for both speakers, you can then go through the full room correction process, which can deal with the smaller wobbles in frequency response.
Best of luck! I'll be going through this with five Neumann DSP speakers soon!