Josh, as Matt(player8)indicated, SET stands for single-ended triode. As to push-pull, this refers to a type of amplifier operation which is in contrast to single-ended operation and which is sometimes referred to as double-ended, although push-pull is the more common terminology. It should be noted that either transistors or tubes can be used in push-pull or single ended configurations and in the various classes of operation(A, AB etc.). The single-ended type uses in its simplest form just one output transistor or tube. The push-pull uses at least two output transistors or tubes which operate in parallel and are fed voltages which are equal but have been inverted 180 degrees in phase. The result is that in effect half is "pushing" while the other is "pulling" in the same direction in a co-operative effort. The push-pull circuitry is more complex but has advantages especially for higher powered amplifiers in matters such as distortion and hum rejection. Push-pull operation tends to cancel out all even orders of harmonic distortion, while single-ended operation can develop a significant amount of second-order harmonic distortion in particular. As you mentioned, this distortion can be used by guitar players and actually becomes an important part of the sound of the instrument. For home audio, however, accuracy is desirable(amplifiers shouldn't become musical instruments)and applying attractive terms such as "sweet", "warm", "full" etc. to distortion isn't really helpful. Note the test results(including response fluctuations resulting from the high output impedance)of a fairly typical SET amp
here , selling for $4,000 per pair.
If you're interested in studying this, of course there're entire books on amplifier design available at the library. The St.Andrews U. site has 3 pages describing some of these factors
here and Doug Self also has a
site covering amps and other interesting audio topics.
