The SET amp question.

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).

I have had 45, 2A3, PX-25, 300B, 211, 845, and 805 SET amps.

As others have noted, with your speakers, your options are completely open - and IMO the 45 cannot be beat all-around. Either the Yamamoto or the Fi (the X is good but not as good as his stereo or monos; the X is the budget version).

The 45 tube will amaze you with its rock-solid bass, perfectly neutral tonality, amazing sound-staging, and complete freedom from any weakness at all, really.