When people hear some of my restored radios or record players, they comment about it having a warm "tube sound". Is this just a subjective term? I don't know that the sound is actually warmer, I attribute it to the fact that the limitations of some of these old tubes and amp designs were limited on the high frequency end.

Actually, I think it's because tube amps/preamps don't have the instant-switching sharp "edges" in the output waveforms that solid-state devices have. A stylus tracking vinyl has the same rounding out of sounds, VS the harsh edges of digital sources. It's amazing how much is lost with digital sampling these days. I suppose if sonic accuracy is the goal, solid-state/digital sources are the most accurate. But our ears aren't linear devices either; the smoothing that vinyl/tubes provides is much easier on our ears, and for the most part is far more pleasant to listen to. In that regard, a bass-reflex or infinite baffle speaker system will always sound smoother than a high-tech in-your-face system, which is so popular today.

Just my $.02 worth. (Why don't computers have a "cents" key? My Underwood does . . . )

:Just my $.02 worth. (Why don't computers have a "cents" key? My Underwood does . . . )
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I too long for the CENT key.
Maybe if Underwood made some computers, we'd have one.

It's definitely a subjective term. Some people find that the distortion/harmonics added by tube amplification is alluring; others do not. Personally, I like the "tube sound" but also enjoy the superbly flat reproduction that solid-state/digital amplification offers as well. This may be age-ism (I'm old, so don't call me a bigot) but I think one's preference in this present day depends on his or her age- as people grow old their hearing naturally loses high-frequency response and they tend to favor bass-accented sound.

Being a guitar player from way back, that hobby is now full of buzz-words that try to describe the tube amp sound.
Guys post and argue in forums about which new or old 12AX7 is the best 'sounding'...it's all subjective. I have tube amps that will make your ears feel like they have ice picks in them from the excessive treble..and like wise have some tube amps have limited top end due to many factors and sound 'warm' compared to others.
They even have a 'brown' sound in amps..it never ends.