Single ended means that a single active device(or paralleled set of active devices) handles the signal. The active device could be either a tube or a transistor.
The alternative to single ended operation is push-pull, where one active device (or paralleled set of active devices) handles the positive parts of the waveform and another handles the negative parts of the waveform.
For audio use, a single ended stage must run class A. A push-pull stage can run class A or class AB, depending on biasing. In pure class A operation, neither device in the push pull pair ever shuts off.
In class AB operation neither device in the push pull pair shuts off on small signals but on larger signals one device is off while the other is processing its half of the waveform. Effectively the amp runs class A for small signals and transistions towards class B as signal strength increases.
The higher the bias (and the hotter the amp runs) the longer the amp stays in class A before moving into class AB as the signal increases.
Pure Class A amps are inefficient and draw twice their rated power from AC line constantly. The power draw is independent of the amp's output and most comes back as heat. They are space heaters.
As I type this I'm listening to a pair of Pass Aleph 1.2 single ended mosfet monoblocks. Each is pulling about 500 watts from the AC line and the room temperature has risen 8 degrees in the last 2 hours. But they sound divine!
I vote for Pass also.
