[Elecraft] KIO3B - different audio levels?

[Udo Langenohl - DK5YA]

I've upgraded both of my K3 while ago with all parts available to make 
them K3S (more or less).

My main K3 shows some weird behavior with the audio level comparing left 
and right stereo channel (KIO3B). I'm feeding this K3 mainly with 144 
transverter signals from XPol yagis (moonbounce system) with the Hpol 
signals on the main RX and Vpol signals at the second RX (AUX). Main RX 
is left channel on the USB soundcard while SUB RX is the right channel 
(Windows 10, MIC input).

Even when on both RX (main and sub) the HF signal level is exactly the 
same (i.e. -120dBm from a HP8664A signal generator, split by 3-dB for 
main and sub) the left audio channel (main RX) shows exactly 6dB less 
compared to the right audio channel (sub RX) at USB audio out. On the 
IF-side (28 MHz) signal strength is exactly the same level, measured 
with my Siglent Spec Analyzer. Just .34dB difference main/sub.

This doesn't happen on my 2nd K3 which is almost exactly the same inside 
like #1 except it's a 10W version.

Any idea?

Udo, DK5YA