Flatline's 175 is not very sensitive, 2.83V RMS of B-weighted noise raising an estimated sound-pressure level of just 82dB at 1m. This is much lower than the specified 88dB, and might contribute to RN's finding that the speakers lacked "jump factor." However, to some extent this will be compensated for by the ribbon's cylindrical-wavefront dispersion: the level drops with distance according to more of a 1/d law than the 1/d2 typical of point-source radiators.
The ribbon's impedance, shown separately on the right-hand side of fig.1, drops to a value of 2.4…