I'm not an amp designer, so all I can say is that the SET amps I've used had a high output impedance that affected the speakers response considerably and those with feedback had less with correspondingly less impact on the speaker's performance. OTL amps OTOH have so little as to be insignificant, ergo I assume the 'T' amps fall into this category. In the latter case, the speaker's Qt + any passive components resistance is all that needs to be accounted for.
GM
GM