SET's are by far the worst and in my opinion are not suitable for hifi use (with few exceptions).
It all revolves around a few main areas... Feedback being the most important. The more feedback used, the lower the output impedance (and hence lower the response errors into different loads and greater the damping factor) and flatter the response. The amount of feedback that can be used depends on the design of the amp and, most importantly, on the design and quality of the output transformer. You cannot just keep arbitrarily increasing feedback as the amp will become unstable and turn into an oscillator.... and the quality of the OPT is usually the biggest limiting factor in how much can be used.
Radford were about the most technically advanced valve amps in this respect (still today!). They had superb OPT's and mainly used a pentode/triode phase splitter which reduced internal phase shift still further and allowed something like 35dB of feedback. This is a lot by valve amp standards and is why the Radford amps have such low distortion figures and wide frequency response. Around 20 - 24dB is more common (remember dB is a logarithmic scale!) and these days less still or non at all can be found in use... which some like the sound of but it turns the amp into a colouration machine/tone control in truth!
The type of output valve also has an effect on all this but it gets more technical and most of the more common verities (EL84, EL34, 6L6, KT88, 6550 etc) are much of a muchness in this area. Far more important is whether they are used as pentode, ultra linear or triode.
Speakers wise well the more the reactance and the more the impedance changes with frequency then the more important it is to have a low output impedance from the amp driving them. Complex crossovers and multi drivers can be worst in all this but... not always!

Ah, ok.

I think the whole synergy thing is overlooked in objective-land and this is one area I think where going active may not be 'automatically' better, especially if the passive crossovers have been very carefully designed around the drivers used. It was different in the 80's Linn/Naim days where the basic speakers had such terrible flaws that going active could only make them better, but today, with modern drivers which have no nasties in them (this rules out most metal cone drivers which take off like tin cans in the treble), the crossover's job seems far less intrusive.

Just my current musings and easily open to challenge

It's very true. I have heard only one SET amp that would not be blown out of the water by a NAD 3020. And that one would of course be much better if it had been push pull and used feedback.
An amplifier should not have to be "used with the right loudspeakers"! Anathema! A policy of "first choose an amp with 5% distortion and a frequency response that looks like a contour map of the Pyrenees and then try and find an incredibly inaccurate speaker, that has opposite faults to the amp, in order to get some sort of "balance" to the sound" does not appeal!!

In the days when the valve ruled supreme, single ended power amps were quite rightly regarded as suitable only for table top radios and the amps in TV sets. Anything with any pretence towards quality used push pull.

The rantings of 6 Moons and loads of suspect hifi journalists do not dilute the technical facts honed over the last 100 years!
I find pathetic the whole camp of "lets take parts and ideas from 1922 and see if we can boost the distortion up to 10% and get only 100Hz to 8KHz frequency response, maybe we can get only 5 watts as well if we try.. then we'll make stupid claims about "musicallity" and "purity" "

It's like taking a Model T Ford and trying to claim that because it only does 35MPH, breaks down often, has no roof and a bone shaking ride that it's the essence of true motoring and far superior to modern cars or some such...

Jaz can you watch how you debate your point I know you feel a bit provoked right now.
This is a interesting thread lets keep it that way

My experience is that, more often than not, high sensitivity speakers and low power valves can be fun but also very coloured.

I used to own (well I still own them, I'm just not using them now) speakers that used just a capacitor in series with the tweeter - so as minimal as they come. I now use a speaker with second order slopes for the mid and treble and third order active for the bass. Contrary to the minimal crossover concept the more complex crossover speakers are better in every way (they are even more efficient).

In post #9 DSJR states that modern drivers have 'no nasties in them', but that just isn't true and nor is the following generalisation that metal domes 'take off like tin cans in the treble'. The push with a lot of drive unit technology has been to make the driver as pistonic as possible over its intended operating range. The counter to this is that breakup is often quite energetic outside of the expected usage range and if not crossed over quite steeply, or with the addition of notch filters, there can be plenty of 'nasties' present.