@carlsbad @charles1dad
It does seem like transformers are a bit far away from the OP’s question.
I went there because there was an assumption that a fairly narrow range of speakers are tube-amp friendly.
What I think has been established is that there is
(a) a wider range of speakers friendly to tubes than assumed.
(b) a need to understand what a specific speaker’s performance is regarding impedance
(c) a need to understand the capability of a given tube amps' performance.
My comment about transformers relates to (c) which bears on (a).
In other words, if the OP had found a speaker with reasonable impedance performance, then the question would become how well can a given amp perform? Part of answering that question would depend on (c).
Of course, the OP may be perfectly happy or stuck with their amp, in which case they could assess the quality of the transformer on it in order to see how variable their speaker choices are.