Why do Tube Amps sound more romantic v SS amps

I have heard that most speaker designs are built to deal with solid-state amplifiers. Add that buying a tube amp does not flourish with modern day speakers. Wish I could recall specifics as to why this is. Maybe someone else does.

When you see 4 ohm nominal speakers with average to low efficiency (~90dB a Watt and below) - yeah, those are more likely to have been designed for SS. It’s not that tube amps can’t handle 4 ohms, it’s just the practical matter that most SS amps significantly increase their rated power output when you go from 8 ohm to 4 ohm loads. Tube amps have a hard time keeping up with that (tube Watts are NOT more powerful than SS Watts), so if the speaker maker knows his market he’s probably going to design more with SS in mind.

There's a wide spectrum of sonics & performance in both SS and tube amps. You have to get specific about topologies and models to make any meaningful generalizations other than @atmasphere's (Ralph) very interesting and smart categorization of power paradigm (tubes) versus voltage paradigm (solid state) amps. Sometimes a speaker design will prefer one or the other. First research your speaker and audiophiles' experiences with amp parings. 

My Tannoys are in the middle - they can thrive on SS or tubes. I have a Phison A2.120SE that really turned me around on solid state amps. It's not "romantic" per say, but it's surprisingly sweeter and smoother sounding than the Rogue Audio tube amps - while providing the incredible speed an separation. I didn't know SS (class AB even) could have a midrange like this. The my VAC200iQ tube amps (a pair run as monos) are voiced to the "romantic" side but still very powerful, extended, and detailed. They're sweeter than the Phison, but only just. Then the Rogue Apollo Darks (250 tube Watts per side) excel at macro dynamics and slam; you might mistake them for SS given most of their sonic attributes.

In the past I've also had classic (restored) vintage tube amps - Heathkit W4, W5, Eico HF-87. And those are wonderful classic tube sound - but here too there are significant sonic differences between models. Romantic beautiful tone, fun sound, and they don't particularly sound rolled off either.