[Elecraft] K3 RX Audio Roll-off below 400 Hz

[Bob]

My K3 is in the 61XX serial number range (almost 2 years old), and I'm
running FW 4.67/2.81.

While setting up WSJT-X, I've noticed a rather pronounced audio roll-off on
the waterfall display.  By other means I've verified that the sound card
being used is flat down to below 20 Hz, so it isn't that.  Assuming that it
is an attribute of the K3 itself, things are flat down to 400 Hz.  Then
there is a -3 dB roll-off at 300 Hz, -10 dB at 200 Hz, and -20 dB at 100 Hz.

At the moment, my widest filter is an 8-pole 2.8 KHz (A 6 KHz AM filter is
on order), and it properly centered and wide open.

Is this roll-off similar to what other folks are seeing?

73, Bob, WB4SON