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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