The SET amp question.

The short answer-SET's amplify the whole wave form with one tube and push pull splits the signal in half (positive & negative) and one tube amplifies the neg and one tube amplifies the pos and then its put back together again.I know that somebody with more knowledge will chime in and really explain things!

The DIY HiFi Supply offerings are high quality.

Racamuti got it mostly right, but actually a single output device is not mandated in a single-ended design, though it's certainly the most common situation.

"Single-ended" implies that there is no phase-splitter - that the output device(s) amplify the entire audio signal, positive and negative. The corollary to SE is push-pull - an amp that is one cannot be the other. However, more than one output device can be used in SE topology - this is referred to as parallel single-ended. Some amps of this variety can sound extremely good with no perceptible weaknesses compared to a single output device design.

Single-ended amps are also, by definition, class A, since the output devices must be pulling current 100% of the time.

In addition to SET - single-ended triode - perhaps also consider single-ended pentode or even some single-ended transistor amps (Pass). I, and many others, I think, consider the most important element of SET to be the first two letters of the acronym - triodes may offer the most linear amplification curves, and thus the simplest, lowest/no feedback circuits, but the more complex tubes can offer a very compelling sonic presentation as well. I'm thinking of the Almarro amps (which I have heard) and Audiopax (which I have not).