Rrog is correct that image dispersion along the vertical axis is sensitive to high frequency information. In fact see my posts dated 5-3-13 in
this thread.
However, based on the photos in your system description it appears that you are probably referring to a short length of XLR interconnect used between the DAC and the integrated amplifier. If so, I find it hard to believe that it would have such profound effects on vertical image dispersion. Therefore I suspect that Mofi's theory is correct.
Perhaps one of the two XLR cables has pin 2 at one end connected to pin 3 at the other end, and vice versa, which would put the two channels out of phase with each other, instead of having pin 2 connected to pin 2, and pin 3 connected to pin 3. You can check that with a multimeter.
Regards,
-- Al