Just got a new power amp

@cleeds   This amplifier has only balanced input.  Circuitry between input and output is not balanced and output is not balanced.  Output is single-ended, with Mosfet bridge to switch "flip" direction of the speaker.

@cleeds   Input stage can be either "balanced" or "true balanced".  "True" refers to inputs that are not referenced to GND, like in instrumentation amp or transformer.
When pre output and amp input are both "true" balanced signal to shield capacitance in cable disappears, since signal is not referenced to GND.  Example of such configuration is output transformer to input transformer.

Amplifier can have balanced inputs or balanced outputs (to increase power), but "Fully Balanced Amplifier" means that balanced input section (instrumentation amp or transformer) is followed by two separate amplifiers, each driving one speaker terminal.  Such configuration helps to improve common mode noise rejection and reduces even harmonics.

That is at least how I understand it.  Other terms like "fully balanced input" are also used, not to mention "rms power".  Most of the time I understand what they mean.
 

The PS audio s300 is supposed to have balanced inputs and outputs, but it isn't quad differential.

@invalid  S300 output is supplied from single supply only.  To achieve net zero output at 50% duty cycle speaker is "flipped" between V+ and GND by bridge of 4 Mosfet transistors.  Some modules achieve it with dual +/- supply and only 2 Mosfets.  To me it is more "bridged" than "balanced", but it doesn't matter, as long as we know what it is.

kijanki

"Fully Balanced Amplifier" means that balanced input section (instrumentation amp or transformer) is followed by two separate amplifiers, each driving one speaker terminal.

I understand that intuitively what you wrote seems "true," that it "feels" like it's correct. But it is completely false. You don’t know what you’re talking about. Here’s a guy who actually understands balanced circuits and who notes the myth of your claim. (I suspect you’ll reject this,  just as you have the information I’ve quoted from other manufacturers. I’m including it for the benefit of others):

Balanced line equipment is often also fully differential, like our amps and preamps. A differential amplifier has two inputs, and in theory only amplifies what is different between those two inputs ... A differential circuit isn’t double the number of parts of a single-ended circuit, although it does have more parts.