Just got a new power amp

@atmasphere   Amplifier in question PS Audio S300 has only balanced input with single ended stages following.  I don’t believe that adding, for instance input transformer, makes any amplifier "Fully Balanced".  In my opinion such amp is an amplifier with true balanced input.

@kijanki Did you not mean to use 'S300' twice in the above post?

@atmasphere   I replaced second "S300" with "such amplifier" to make it general.  To me something that is "Fully Balanced" has all stages, including output stage, balanced.  One criteria of balanced output is lack of GND return current.

@atmasphere 

I'd put it the other way 'round. We audiophiles are very used to hearing interconnect cable differences. But recording engineers, who use balanced lines in a studio, are not. When the balanced line system is set up properly (as usually seen in a studio) the 'sound' of different interconnect cables goes away. 

Agreed.  I was not limiting my "something" to the physical cable.  It could include associated equipment and/or the observer.

One criteria of balanced output is lack of GND return current.

@kijanki So if one side is at ground, but there is no ground return current to any other part of the amp, your criteria is satisfied? Or is the fact that the speaker circuit is not complete without a ground connection a problem?

Audio Research used to ground the 4 Ohm tap of their output transformers rather than the common tap, so they could cathode cross couple the output tubes; IOW the cathodes of one half of the amp were tied to the common tap and the 16 Ohm tap was tied to the other half of the amp's cathodes, as a feedback mechanism. To do this obviously the feedback was balanced. Does this meet your criteria?