The crossover frequencies on the BMR Tower are at 850 and 3,800 Hz. Between those two frequencies, the two mid-range BMR drivers are in use. It is worth remembering that in the BMR Tower, these mid-range drivers are arranged in MTM fashion, above & below the ribbon tweeter.
James Larson showed the losses in SPL he measured as he moved the microphone above (+degrees) and below (-degrees) the tweeter axis (see below). Those losses are in the frequency range of ~1.7 to 4-5 kHz, with the greatest loss shown at just above 2 kHz (see the -15° trace). This frequency range is covered by the mid-range drivers – not the ribbon tweeter.
These losses are just what you would expect with two mid-range drivers arranged above and below a tweeter as an MTM. As you move a microphone or your ears higher or lower, you will notice a loss in the response because the relative distance between the two mid-range drivers changes. Those changes in distance result in changes in phase relationships. As you move further off-axis, you eventually get to point where the changed phases between the two mid drivers results in cancellations. The amount of loss varies with the physical distance between the two mid drivers and the frequencies tested.
The vertical off-axis response of all MTM speakers will do this. It has nothing to do with the tweeter.