The unbalancing I was thinking of was applying a different load impedance to + and - output signal legs, but losing one of two active outputs could result in a drop of 6 dB.
JR
I agree, and, also... one of method for unbalancing balanced lines implies connecting (-) contact go ground on input side, and if existing output isn't truly transformer coupled, but simplest electronically "balanced" (two independent buffer circuits, not something based on G. D. Pontis design described in
Hewlett Packard journal, 1980, p.12), there will be 6dB drop of signal level for sure, and one of buffer outputs will be shorted to ground, despite all.
To check this, its needed to test whether pin 3 and pin 1 are shorted on input side.
Some workaround for this problem may be transformer coupled adapter (Lundahl or similar)
More about unbalancing balanced lines and interconnection cables can be found on
Rane Note 101