does this mean that being push-pull or class AB, st70's don't sound as good as SET's ?
to me, yes. but it does not mean the ST-70 or any push pull amp is not good. FWIW, i have owned an Anthem Amp 1 40wpc ultralinear push pull.
6550 SE it is!
the 6550 has a maximum plate dissipation of 35 watts, and in Class A (single ended) that means, with a
theoretical maximum of 50% efficiency, you will get 17.5 watts with more than 15% total harmonic distortion.
the 6550-A has a maximum plate dissipation of 42 watts and can yield, in Class A (again, single ended) 12.5 watts at 7% total harmonic distortion or about 20 watts at 13.5% total harmonic distortion. of course, if you like less distortion, you have to go lower wattage.
a typical Class A operation is generally accepted (debatable) to be 25% of plate dissipation, to get acceptable (highly subjective) distortion figures, in single-ended terms.
i thought you ought to know, and for the benefit of the others too.
i apologize if this is not of help to you.