What are the ups and downs of both?
In the end, its all about your investment. Tubes are hotter, and the power they make watt for watt is more expensive.
If the closest you can get to real music is your goal, tubes are the way to go. If its anything else (sound pressure, low heat), probably solid state.
The reason is simple: the ear perceives sound pressure via the presence of higher ordered harmonics- the 5th and above. It does **not** gauge the fundamental tones! This is probably due to the fact that pure fundamental tones are very rare in nature.
Tubes might seem to make more distortion when you look at amp specs on paper, but the harmonic distortion they make tends to be the lower orders- the 2nd - 4th, to which the ear is far less sensitive!
So a simple way to put it is that tubes more closely obey the most important rule of human hearing which is how we sense sound pressure. So it is much easier to build an amplifier that sounds natural to the human ear with tubes.
Another way to look at this is that the ear hears volume on a logarithmic scale. In a way, it comes quite close to sensing harmonics on a log scale too- being so sensitive to the higher harmonics that they can easily be detected by the ear even when the THD is only 0.005%.
We don't weight the harmonic distortions electronics make under our current measurement regime. The 2nd order (which contributes to 'richness') is given the same importance as the 7th! This is quite odd, because we've known since the 1930s that the ear is more sensitive to higher orders (see the Radiotron Designer's Handbook, 3rd edition). In the 1960s, General Electric did a study confirming this, that showed that humans don't care if up to 30% of 2nd order is present, but object quite a lot of 0.1% of the 7th order is present. Yet despite having this knowledge, we continue to ignore the implications (probably because of the money; transistor amps are simply less expensive to make and often have higher profit margins).
This is why there is a solid state/tubes debate; its why tubes are still around after being declared 'obsolete' in the 1960s; and the debate won't be going away any time soon. When our testing systems continue to ignore some of the most important aspects of human hearing/perceptual rules, you can count on the spec sheet not telling you how the equipment sounds (or doesn't) just as spec sheets have done for the last 60 years.
