charles1dad,
A nice concise history of what happened. I would add that another aspect to the story. When stereo came around, the need to find room for two speakers, instead of just one, meant that smaller speakers gained in popularity. These smaller speakers, particularly of the air suspension (sealed box) variety were inefficient and required higher powered amps and this favored solid state over tubes.
Mozartfan,
There are many pentode/tetrode tube amps running pushpull that are extremely musical and great sounding with horn systems, fullrange driver systems, and multiway systems utilizing fullrange drivers. You cannot generalize from hearing only something like the Defy 7 and certainly not by "listening" to YouTube videos. I have heard terrific 6L6 and KT66 amps and some of my favorites are 350B and 349 amps (the favorite of the amps I own runs the 349 tube). I also own a pushpull 45 amp (triode tube) and a pair of Audio Note Kageki (parallel 2a3 SET). All of these amps sound good. My all time favorite amp is an output transformerless amp that is a custom built amp, and a pushpull Western Electric 59A amp (252 tube) which is something I could never own as a pair is well into six figures. Even with tubes that I don't particularly like (KT88, KT120, KT150), there are examples of amps that run them that I found to be decent. I know that there is a strong desire to come to conclusions about various aspects of design or parts choice, but, it really is not that simple to do. The more I hear, the LESS I am able to draw definitive conclusions in these areas, although I do see rough tendencies. As to such rough generalizations, I do like quite a few speaker designs that use fullrange drivers in multiway systems (e.g., Cube Audio Nenuphar Basis), a small handful of true single driver systems (Charney Audio, Voxativ), but mostly, I like horn-based systems.