Dave,
As to your first question, the main differences between different SET amps ARE the output drivers employed. There are many other differences in design (input stages, power supply, transformers) but by-and-large the majority of the differences of these amps comes from their output tube. The majority of SETs employ (like you said )300Bs, 45s, 2A3s, 845s, and a few others.
SET works by pushing the entire signal through one tube, without splitting the signal into it's components (positive and negative). This is the "single ended" part (although that's a very simplistic explanation) of the SET. The Triode part is simply how the tube is wired to run. A triode tube consists of just three parts (the anode, cathode, and grid) while pentode tubes have five.
Push-Pull differs (we'll talk about triode p-p) by splitting the signal into positive and negative components, and then having one tube "push" the positive side while the other tube "pulls" the negative. These "doubly" amplified signals are then recombined, creating more power.
Push-Pull has some advantages. More power is generated. Many harmonics are cancelled out when the signal is recombined (which won't be a perfect recombination). SET, though, keeps the signal MUCH more in tact, but you hear more harmonics. I believe that SETs have much more "magic" than push-pull, YMMV.
Your question about the tubes is fairly easy to explain. Think of different tubes like different engines. They are all designed to do different things (the majority of which is NOT high-end audio). Some output much more power, other do it cleaner. You can push a tube to make more power (like an engine), but at the cost of reliability and operating life. All these tubes were designed for a specific purpose at one point, now us 'philes are making them do our bidding. Every tube has a "sound." Just listen and find which sound you like. Hope this is a little help. Cheers!
Brian
