I have owned just about all the Flex Radio's from the SDR 1000 to the 5000A (as well as the 3000 and 1500)
I acquired the 6300 about a month or so ago and find that the NB is not all that effective. It does work to a degree, but not as effective as in PowerSDR.
However, when activated on a strong signal as soon as the station starts transmitting the audio is badly distorted and then after a few seconds normalises
I have tried various setting with little or no effect
I wonder if others are experiencing the same problem