[Elecraft] KX-1 receive passband - any adjustments possible?

[Barry N1EU]

The BFO offset in the KX-1 is fixed at 600hz.  I would hope that the
receive audio passband should be somewhat centered around 600hz.

I recently received KX-1 s/n 2550.  I quickly noticed that the passband
seemed "high" when compared to an older KX-1 I was previously using.  I
looked at the audio from both KX-1's on the pc and confirmed that with
about the same selectivity setting on the front panel, the old KX-1 peak
passband was 350-770hz (okay) and the new KX-1 peak passband was 650-1100hz
(not good).

How would one go about correcting the KX-1 passband?

Thanks & 73,
Barry N1EU