So making general statements means very little unless you have the experience and knowledge in the specific area
So who’s doing this ? People have rightly posted that speaker impedance and phase angle characteristics are factors that require awareness and consideration. This is true as they greatly influence how successfully a tube amplifier will pair with them.
No one has disputed that there are tube amplifiers capable of managing difficult/challenging speaker loads. However, the question of the OP was what makes a given speaker “tube friendly “. People have responded in kind.
Charles