i think a largely related point is that most modern speakers are designed with the assumption that a ss amp is going to be driving them, i.e. that the output impedance of amp driving the speakers is near zero, or said another way, the dampening factor is above 20. the crossovers are designed with that assumption.
so tube amps then often sound bloomy, tubey, or rolled off unless they employ lots of feedback. but if you ask me, feedback defeats the purpose of using tubes in the first place and trades away benign lower order harmonic distortion for irritable higher order distortion. i think this is primarily the ss sound, but there are exceptions.
tubes are inherently linear (most used for audio) and don't need much if any feedback to obtain good results if designed carefully, but the rub is their output impedance will be nowhere near zero. then it is a matter of whether the speakers are designed for such an amp. it is competely possible to design the speakers for said amps and have tight taut bass and linear response while maintaining the purity of sound obtained from an amp lacking higher order distortion.
the problem is there aren't many commercial speakers designed in such a way. you are left making your own or having custom speakers made for you to your specification. this is the beauty of diy, being able to design your total system for the sound not often obtained through commercial means, imo.
there are some commercial tube amps though that do employ modest amounts of feedback to obtain necessary dampening while carefully designing the circuit for distortion cancellation to balance out the spectral content, leaving a purer sounding amp and keeping some of the benefits of SS amps. i believe my Audioprism Debut amp to be one of these examples. It doesn't sound like a SET amp but it does have many qualities not often found in commercial SS or tube amps.