There are experts here who will correct me if I'm wrong, but SET mode involves quite a lot of distortion, but I suspect this distortion is not smeared over time as it can be with other methods. Worst case "time-smearing" being with many Japanese "high spec/high global feedback" amps of the mid seventies, which couldn't reproduce fine details and reverb properly even when trying..(OK, there were many other reasons too, but I bet that lots of global feedback judiciously applied to bring the THD right down contributed loads to the lack of sound quality).