Ultralinear vs. Triode vs. SET

Well...I think it's a bit more complicated than Philefreak indicates, but I'll say first that generally, I LOVE the sounds of SET amps.

SET amps definitely have limitations...in power and damping factor, mainly. Most SETs are low in power--that's where the term 'flea-powered' comes from, and I LOVE it! That means that most are limited to single-digit number of Watts. Larger tubes such as the 845, the 805, and the GM70 (and a few more, of course) are required to produce more than the 8 Watts or so that a 300B output tube produces.

BUT...BUT...BUT...good SET amps reproduce music that sounds simply more like music rather than reproduced music. I've owned 4 stereo or pairs of mono SET amps, and I'll never go back. Some say that SETs can't sound great driving lo-impedance or lo-sensitivity speakers. They need to come to my house, where I'm driving the top-8 octaves of 89dB-(in)sensitive, 4-Ohm-rated Audio Physic Avanti speakers with TWELVE-Watt SET amps, and the combination sounds not simply better than it has any right to but unqualifiably excellent.

I'm not a golden-eared audiofile in that I cannot recognize the dozens of tiny differences among pieces of audio equipment that the GEAs can hear and describe. My approach is longer-termed and I deal with overall sound character...a macro approach, one might say.

You might benefit from reading Art Dudley's 2004 review of the Antique Sound Lab Explorer 805 50-watt SET amps.

... http://www.stereophile.com/content/antique-sound-lab-explorer-805-dt-monoblock-power-amplifier ...

I read that in 2004, and having the same Quad 989, lo-sensitivity speakers Art had, I bought a pair of ASL805s sound unheard. I LOVED them and, after going thru even-less-sensitive speakers, and high-power push-pull-triode amps, and hi-sensitivity speakers, I'm back to SETs and have just ordered my 2nd pair of ASL 805 amps.

The music just sounds better. ... :-)
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If your Cronus already provides enough power for you (55 watts?) and you wish to try SET, get Audio Mirror 45 watt SET monoblocks. Nearly the same power as your Cronus, but SET! Glorious amps.

" From both a technical and sonic perspective, what are the differences between a tube amp with switchable ultralinear and triode mode vs. a true SET amplifier?"

A tube amp with a UL/Triode switch will most likely be using a pentode. The switch makes it operate "like" a triode but it is what it is and some may like the sound of a KT88 or EL34 wired that way but it's not a true triode. A true "SET" amp will probably be using a 45,2a3 or 300b tube each having their own flavor/character and power output.

That's kinda general but the best I could do per your question and keep it simple.

I do think anyone who loves music should at least give a listen to a "true" SET mated once in their life. Hopefully it will be mated to a speaker it likes.

Ooops, was rushing at the end of my post. Last paragraph, delete "mated" after SET.

Very simply: a pentode based amplifier will be more versatile with speakers. A SET is very finicky and will like some speakers and music more than others. Despite the poor measured performance of the SET, it somehow manages to sound more realistic and more believable than any other amplifier type ... provided it is matched with the right speakers and the right type of music.