Solid-state VS Valve Amplification
The main advantage of a solid-state amplifier is that it uses "negative feedback" a feedback that literally fed back into the circuit - to stabilise the signal and improve gain (or volume). While this gives you plenty of power to drive massive speakers and hurt your neighbours, it does affect the characteristics of the audio signal.
Valve amps, on the other hand, use very little negative feedback and have very linear circuits. If your aim in amplifying an audio signal is to alter it as little as possible from the original , then a linear circuit has a good chance of doing that.
Something that baffles many valve n00bs is the apparent "weakness" of valve amps, in terms of power. An example of one called the PrimaLuna ProLogue FouR has only 35W per channel! What a piece of junk! But no: because valve amps are linear, their wattage "counts" for a lot more. About eight times more. So the ProLogue FouR can pump it with a 280W solid-state amp.
What is then true is that, dollar for dollar, it is much cheaper to get "valve sound" using a valve amp than it is to buy a very high-end solid-state system.