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 can't explain anything about it technically, but if you have not heard a SET or SEP on your hi-efficiency speakers you owe it to yourself to have a demo. The sound can be intoxicating.

At 110 dB, unless you have a huge room to fill to orchestral live levels, you have the entire universe of SE designs to try. From the 2-3 watt 2A3 based to 9-10 wpc 300bs on up to the 50 wpc Kronzillas. Lots of great amps to try in that range.
Tell us your room size, music type, volume level and budget and we can make recommendations.

Unlike some of the responses, I cannot lend further technical explanations, but I can share some impressions of how single ended triode (yes, the "T" is in place in my experiences I'm sharing here) distinguish themselves in presenting music. I have been listening with a pair of 300B SET amps for about nine years now. I've strayed but always seem to come back to them because they seem to suit the music I prefer and the way I prefer to hear it. These impressions are my own and YMMV. SET seem to excel to extremes in breathing life into vocals and more sparse and simple instrumental arrangements. When the music starts to get more complex and layered and dynamic, SET can be bested by other forms of amplification, though it does OK for the most part. If you listen to a whole lot of music of that latter type, you might be disappointed overall. You most certainly will not be disappointed if your tastes run to the former (more simple, sparse arrangements). IMO, No other form of amplification seems to breath life and presence into music the way SET does at its best. The term "Living Presence" comes to mind. They excel at soundstaging - creating an illusion of a three dimensional space. Your sensitive speakers may indeed be well suited, but sensitivity is only part of the story. A flat impedance curve will also go a long way towards making the combination work out well. SET amps generally will not have much authority in the bass, nor as much sparkle the highs, but MY OH MY they do midrange, where 90% of the music is at, like nobody's business! A cheap introduction to the power of SET can be had in spades with the Wright 3.5 amps that use 2A3 tubes. I've seen them frequently on the used market for $800-900 for a pair of monoblocks. They are all about midrange, but that midrange is like crack to a junkie if you appreciate it. They are SET at the extreme end of the spectrum. Again, if you are a hard rocker or into heavy orchestral music or opera...you get the picture...you probably should look elsewhere. That's just my opinion. I keep coming back to SET myself, but I would not call it a versatile performer. It is an eccentric...more of a one-trick pony than a crowd pleaser for the masses. Another intoxicating alternative which has more potential for a more linear presentation, and is more versatile at presenting a wider range of musical preferences would be OTL amps, such as the excellent offerings by Atmasphere.

One further note on SET - I think pairing the amp off with the right speakers is so much more critical with SET because the wrong combination is just going to be a waste of money and effort, and can be very frustrating in that regard. You may want to seek out some opinions from folks who have paired off the same speakers you have with SET amps or SE amps to focus in on the better potential combinations.

Marco

The only thing I would add is that different tubes have different sounds, over and above how they are applied. To me, 300B's give the classic mid range brilliance, but are less good at freqency extremes. The 845, my favourite, is more neutral and more powerful, will tend to give a few more watts. My 845 puts out 22watts and will drive my speakers at 89db sensitivity, with ease. They do still lack the base grip of SS designs, I will happily sacrifice that.