The beacon CW ident is almost "subliminal hearing" but it is certainly better on an A/B test than the KX3
I have no complaints.
THE ONLY way to check it is on a calibrated signal generator and measure the MDS and S/N ratio, then you can compare the readings, when I get a chance I will put my 6400 on the test set and measure it.
For reference I have a K3 a KX3 and an Icom 7100 to compare it with.
The particular preamp setting that results in the best receiver noise floor is immaterial. The available settings are a design decision (such as compensating for bandpass filter loss), and varies by radio. It’s a trade-off.
BTW, high sensitivity affects dynamic range. By your reasoning the Collins 75-S3B (on the Sherwood chart) beats them all. Great rig in its time I’m sure, but looks at its dynamic range! You need to look at the stats in context.
But as was mentioned, if you actually suspect low sensitivity, by all means submit a trouble ticket.
A.J.'s initial comment is spot on: if you own a 6400, you likely want to operate the radio with the preamp at +16dB, and only drop the preamp gain under very noisy or large signal conditions. Otherwise, that really quiet band may have some weak signals you'll never know are there.
I'm on the fence whether to call this a design flaw, but it is important that 6400 owners know that the optimum preamp settings are different for the 6400 than for most rigs. With the right preamp settings, this radio is a delight!
As a Vintage Audio File person, I have the TOP 10 Best all Time FM radio tuners.. This is right off the master FM radio evaluation.. Took me almost 9 years to get all 10, but have about 25 in my collection.... Funny that most of my all time favorites are NOT in the top 10... Not that they dont perform as good as the top 10, but the Looks Of The Tuners and feature set, is what sets them apart...My Favorite is my rare Grundig T-9000 receiver...
Basically I am saying their is So Much More then just the raw specifications of a radio.. But hey,, I am not telling you something you already dont know!!! :-)
Here is a link in case you never visited this site.. These top tuners do sound SO GOOD!!!! Makes digital audio sound DULL!!!
Grundig T-9000 ,, Second best for me, Pioneer F-93 with Space Diversity FM reception..
28.005Mhz Slice A on 10m; Slice B on 75m. CW mode 400hz BW Ant 1 either grounded or connected to 40m dipole E-W at 50 ft. WBN, NB, NR off. SWR seen by Flex ~ 1.7:1 thru tuner. Front end filters in Wide mode. No signal within ~25 khz.
Changing Slice A only; Preamp is common by design to Slice A and B.
Preamp: 0 dbm
Connected -108 dbm ~ 107 dbm
Grounded: -132 dbm
Connected: -124 dbm
Grounded: -140 dbm
Connected: -124 dbm
In my location external band noise seems to limit at -124 dbm. This is a similar situation as frequency is lowered. Above +24 db on 10m on this day and time, additional gain doesn't seem to be useful.
The 6400 is my only Flex so for 6600, 6700, I don't know what they would show, though the same setup and documented results would be interesting.
I ran the same test on my prior Flex 5000 and 6300 but, of course the data was misplaced by the IT staff.
Using a 40m dipole on 15, especially with the EW orientation, the band is generally dead at all preamp settings, a situation that I'm correcting with a NS dipole, I hope.
When I get to 80-40-30 a low noise rx antenna might help; my local suburban/rural noise level is above the ITU curves.
For my use the ability to hear weak stations generally seems adequate but I know it depends on the particular activity and the entire station system. That said, expectations matter, and I can see how it is letting you down in a quiet environment. When I chase spots I do get frustrated at times when others are working the DX and I cannot hear them at all; more a function of the antenna - my working theory anyway.
Enjoy 15, I'll have to wait for higher SSN's. I checked 10m this morning and the band noise is -118 here and could be from many sources, more or less uniform across the spectrum. Solar I guess.