Just got a new power amp

@atmasphere   Tube design is your domain so it is not wise for me to argue, but grounding tap of transformer’s secondary won’t make current return to GND.  Transformer output is balanced, but stage driving primary might be not with one transformer tap returning current to GND.  

I’m sure we can find many exotic configurations that could qualify as Fully Balanced, but PS Audio amp, that started all this, is not.  It is single ended class D amp with differential input stage.  Something should differentiate between this and fully balanced design, like one in the schematic I referred to.   To me it is word "Fully" suggesting, all stages are balanced.

@kijanki Right.

(We introduced a class D amp of our own design about 4 years ago FWIW.)

The question then becomes, in the context of this thread 'is it important?' WRT the thread title, it isn't. Ampex made a tape machine that was used for quite a lot of the recordings made in the so called Golden Age of stereo (1958-1963); that machine had a balanced input and output using transformers but the internal electronics was mostly single-ended. Its use of transformers allowed it to be immune to interconnect cable differences.  

@atmasphere  I agree, "Fully Balanced" design doesn't have to be better - net result is what counts.  There are many imprecise popular or marketing terms in use, why to argue.  I noticed that many great companies still use erroneous "watts RMS" and wonder if it is on purpose.  They likely know it should be just watts or watts average, but that is what most customers think of (product of RMS voltage has to be RMS power) - cannot blame them. 

I've always preferred single ended over XLR.. I think it sounds smoother, more detailed, more analog and has better contact rather than the little pins on XLR...

XLR really is for broadcast studios, military, commercial systems for noise rejection where there are hundreds of cables going hundreds of feet from studio to studio and lots of grounding issues, I've worked soldering racks full of XLR cables that are all super long runs of mogami cable...

That being said, some equipment is specifically designed to sound better with XLR, but my vote is RCA