Paul, I calculate that it would be about a 1.86 times power factor which would be just a little over a 2dB gain. As you know if the 750 Watts were doubled to 1500 Watts it would be a power factor of 2 times, which would be a 3db gain. This being said, On MF and HF a weak signal with a meter signal strength reading of ‘S1’ corresponds to a received power of a ‘-121 dBm’ or ‘0.20mV’ in 50 Ohms and a strong signal with a meter signal strength of ‘S9’ corresponds to a received power of ‘-73 dBm’ or ‘50.15mV’ in 50 Ohms. Most S-Meters are not accurately calibrated and in practice can only provide a relative measure of the received signal strength based on the receiver's AGC voltage.

Some S-Meters are calibrated to read S9 for an input of -73 dBm but do not provide the correct '6 dB' per S-unit correspondence and some only indicate about a 3~4 dB per S-unit. So I would estimate that the increase with only about 2dB gain would show just under 1/2 of an S-unit on a meter that is calibrated for 6dB per S-unit and closer to 1 S-unit on a meter with 3~4 dB per S-unit. Often the correlation between a radio listener's qualitative impression of signal strength and the actual strength of the received signal is poor, because the receiver's AGC holds the audio output fairly constant despite changes in input signal strength.

According to the IARU standards; STANDARDISATION OF S-METER READINGS

1. One S-unit corresponds to a signal level difference of 6 dB.
2. On the bands below 30 MHz a meter deviation of S-9 corresponds to an available power of -73 dBm from a continuous wave signal generator connected to the receiver input terminals.
3. On the bands above 144 MHz this available power shall be -93 dBm.
4. The metering system shall be based on quasi-peak detection with an attack time of 10 msec ± 2 msec and a decay time constant of at least 500 msec.

Respectfully Larry K4RFE