Roughly speaking, SET/Single Ended Triode is class A, Push Pull is class B (probably AB).
With class A you have a single output device which is "turned on half way" at idle. With class B you have two output devices, one pulling up from zero and one pulling down from zero. This allows both devices to be turned off at idle.
Class B generates much less waste heat and can produce more power but you can get little "glitches" around zero, usually called "crossover notch". Class A is terribly inefficient but is totally immune to crossover notch so can in theory give you the best sound.
Class AB is like class B but the output stages are turned on "just a bit" at idle to reduce or eliminate the crossover notch.
SET = low power, hot, expensive, theoretically best sound
Push Pull = like the amps you're used to, more power, more efficient, potentially a bit of IM distortion at low levels but there are ways to design around it
SET has a higher "cool" factor despite the waste heat
