The SPL test was performed under following conditions. The drivers were hung from the balcony. The MLS and gated SINE instrument were used and the voltage supplied to the drivers was 2.83 Volt RMS. (Measured with a Tektronix digital scope with math capabilities.) Since the impedance of the drivers is different, the power applied to the drivers is also different, this explains some of the difference between the B&G and the Carver ribbon driver. For starters, the impedance of the B&G was measured to be 5.2 ohms, the Carver was 4.4 ohms. Results in 1.54 Watt RMS for B&G versus 1.82 Watt RMS for Carver. Please note also that there was no notch filter used in the Carver measurement to suppress the 7 kHz resonance, hence the large difference in that area. The area @ 1.8 kHz needs also work.
I suspect that after equalising both curves the difference between the two drivers may amount on average 3 to 5 dB.