If an amp is running some bias current so that the output transistors are always on but the bias current is (very) low then essentially the amp is biased into class-A. I think that they call these power amps 'sliding class-A power amps' meaning that they have some very low bias current during off-state & the bias increases proportional to the input music signal when it's present.
All transistor amps run some bias current on the output devices at idle. This is not sliding class A, this is AB. Sliding A is where the bias is dynamically changed as the input signal changes.
I was trying to keep the description more simplified, ie. the output tubes are coupled through the transformers so that the speakers see only the transformers.
The transformer transforms the speaker impedance to something that the tubes can handle. Conversely, the tubes are part of the source impedance that the speaker 'sees'.
An implication of this is that a class A tube power amplifier may well be operating in the AB region if the transformer has the wrong impedance on a particular tap, like a 4 ohm load on the 8 ohm tap.
BTW it is not correct to say
SS amps and 4 ohm speakers were made for each other.
The why of it is that all amps, tube or transistor (or class D) make more distortion into 4 ohms as opposed to 8 or 16 ohms. This distortion is audible as increased brightness and loss of detail. Put in a nutshell, if **Sound Quality** is your goal, it is best served by a speaker of 8 ohms or more. If **Sound Pressure** is your goal, there is a weak argument for 4 ohms if you have a transistor amp that can double its power (3db). Put another way, there really isn't an argument for 4 ohms in high end audio, but there might be in sound reinforcement.
