I have personally experienced an epiphany of sorts, while working closely with several different tube and transistor-based preamps and amps. Aside from the typical descriptions, it seems that there is a resolution characteristic that is particular to the better tube preamps and amps. This phenomenon pinpoints certain subtleties in music that tend to get masked, even with the finer SS equipment. In other words, my epiphany was that I could easily detect what were otherwise masked subtleties in musical details that revealed a given recording as obviously "excellent", "good", or "bad" in a way that I hadn't previously observed.
"Flat" is a term that is commonly used to describe SS, although I'm not convinced that it is entirely desirable. Those who mentioned objectionable noise and/or distortion with tube equipment must not realize how that is not typical at all of the better designs. The noise aspect especially should not be tolerated, as it isn't audibly detectable in the competent designs.
Distilling a few thoughts from a fairly general sweep through the posts here incluing a few added concepts:
The better tube audio equipment tends to present a dimension, depth and "air" to music much more so than that compared with the generally flat, or "two-dimensional presentation" that is a classic characteristic or a potential 'complaint' of most SS designs. Exceptions and overlaps of characteristics abound as noted elsewhere, so this is more a matter in the sense of a bottom-line average description. If someone hasn't tried both in a close comparison, they might be unlikely to realize the difference that in fact exists, once a certain quality level is attained for either SS or tube.
Arguments started back and forth about wear failures, yet well-used, 60 plus years old tubes are in operation to this day. Qs ("Q" is a term for a transistor) tend to fail catastrophically rather than "wearing down", or getting weaker, as that is a tube tendency, although Qs suffer from other SS specific types of failures. Q's internal failures tend to be due to microscopic-level breakdowns of the sandwiched crystal material layers, along with some other potential, internal factors. The dreaded "Black leg disease" comes to mind, a type of reactive oxidation or corrosion characteristic that starts on the outside of a Q's lead wires and creeps inside to wreak internal havoc.
Another form of failure could be due to thermal runaway. Most typical Qs are known as Bipolar Junction Transistors or BJTs and these may/will suffer in various degrees from thermal runaway effects. The BJTs' actual thermal failures are generally due to other, external circuitry components' issues as the root cause that setup the Q for failure.
Another Q known as a Mosfet has an edge, as they thermally react in the opposite way from BJTs, so they are 'self-regulating' to a degree. The definition of "wear" comes into play here, as Qs tend to function in their normal capacity parameters for a lifetime, provided all internal and external conditions are favorable, yet failures abound, as noted.
Fact: Both tubes and transistor that run at the extremes of their designated parameters will tend to fail well before those that simply 'idle' at much cooler temps. The same is true for electrolytic capacitors that are located near high heat areas. Tube rectifiers and the heavy-current swinging output tubes run hotter than most all others, so they tend to wear out long before all other tubes.
Fact: Tubes definitely, progressively wear down as their emission levels will become weaker during the amount of use. Some form of minor to major deterioration will be evident over both the operation time and the circuit/app specific degree of 'work' they perform. The wear can be anything from severe in a short period of time, to virtually undetectable after several years or potentially decades of operation.
Fact: Tubes will be hot to varying 'degrees' during operation and generally tend to run hotter than transistors, although plenty of other factors also determine lifespan and wear factors.
Next in line, down from tubes/transistors failures, electrolytic capacitors tend to fail more often than most any other component over years/decades of use.
