Can a Flex6600M use an External 6meter Preamp without adding an External Switch Box(RTR1-A) Box

The analysis above by mikeatthebeach can be misleading since it does not consider the galactic noise which is the dominant noise at 6 meters. I measured the noise figure of my Flex 6600 on 6 meters with a calibrated noise source. This gave NF = 6dB. This is reasonably close to what mikeatthebeach calculated. However when I connect my horizontal 6 meter dipole to the 6600, the measured noise goes up by 30 dB. So even if the NF of the KX3 is better than the 6600, there will be no perceived difference in the received signal to noise ratio of these two great receivers because the galactic noise is so much larger than the receiver noise.