Many SET 'designers' simply buy the parts (including the OPT) and assemble the amp from a circuit they found. The transformer is the tricky bit since the size of the core vs the gap to prevent DC saturation is key to getting bass performance. The larger the transformer, the more critical the winding quality must be to prevent distributed capacitance from limiting high frequency response. They are much harder to build than output transformers for PP tube amps as a result, and usually lack the bandwidth at both extremes compared to PP transformers of the same power rating.
Emphasis added: these statements are false. The latter first: Feedback does not destroy the 'SE sound' if applied properly. It can do it quite readily if improperly applied, which is to say its the designer's fault and not that of feedback if one feels the highlighted statement above to be true.
The former: There's no problem of an amp of high output impedance controlling the cone of the DeVore 10" drivers or for that matter a 15" driver, if the speaker (as in the case of the DeVore) is designed for such amps. More on this topic.
Clearly it is the problem of some SETs whose load lines have become elliptical at the frequencies (bass) that the cone delivers. That is a weakness of SETs (previously explained) and clearly some are better at this than others.
It should be pointed out that no SET can deliver full power at 20Hz (due to a lack of inductance in the OPT; so we see right here that no matter the components, layout, power supply or design of the circuit, that the OPT is paramount). They may well have low power response that low with no problem. But as everyone knows bass frequencies are where most of the power in audio occurs.
