[Elecraft] Lack of low freq response K3 ssb rx audio

[Barry N1EU]

I'm pulling my hair out on this one, but let me keep it real simple.  This
is on a K3s and all hardware/firmware is latest/greatest.

Filter: 6KHz
Mode: USB
BW: 4.00KHz

My question is this:

*Why does the audio passband dramatically roll off in the lower frequencies
as my Shift/FC goes above 1.40Khz?  *


I would think that there wouldn't be any bass rolloff until FC was above
2.0KHz (i.e., half of 4KHz BW).  Instead, what I'm observing is:
    - with FC of 1.6KHz, I see  -8dB at 100Hz
    - with FC of 1.8KHz, I see -15dB at 100Hz

I'm using Line Out to soundcard and Visual Analyser 2014 software.  It
mirrors what I hear in my headphones.

73, Barry N1EU