Just got a new power amp

@kijanki It looks like that amp is. 

But:

I would not use a speaker terminal at ground as your definition, although our OTLs have both speaker terminals ungrounded and floating. 

Many ARC amplifiers do not have a grounded  'common' terminal; the impedance to ground of the common connection is the same as their 16 Ohm connection. 

IOW that is not the defining character of a fully balanced amplifier although it is a characteristic they are likely to have. 

We are experimenting with a tube amp that has a speaker terminal grounded, yet its fully balanced from input to output with dual feedback loops. 

@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.