Tubes typically have FAR higher rail voltages than SS amps, which therefore gives them more dynamic headroom. Tubed amps also clip in a "harmonious" manner, making them less objectionable when overdriven. This means that a smaller tubed amp can play "louder" than one would think, as it has quite a bit of dynamic headroom and doesn't rip your ears / eyes out when it does clip.
The biggest problem with tubed gear is that it typically lacks current and is bandwidth limited, both on top and bottom. The lack of current is what gives most tubed gear that "round, tubby" bass that many folks dislike. At the same time, this "added warmth" tends to "fill out" many of the leaner digital recordings that we hear. The limited bandwidth up top tends to soften the treble response, making hard, bright and edgy digital sound smoother and more listenable.
Like any other amplifier, you want to look for a design that utilizes very large core transformers. With tubed gear, the output transformers are as important ( maybe moreso ) as the power supply transformer. As such, look at the build quality of the amp more than the actual power rating, as most tubed power ratings are "bunk" anyhow. If you applied the same standards for measuring distortion in tubed gear as one does to SS gear, most of these tubed amps would be rated at about 1/4 to 1/3 of what the manufacturer advertises.
There are obviously designs / products that fall outside of these basic recommendations i.e. output transformerless ( OTL ) designs, digital power supplies, etc.. These are all non-standard designs, so the "rules" are apt to be different for these products. In such cases, proceed with caution and just make sure that you're dealing with a reputable manufacturer. Sean
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PS... Buy more than what you think you need, as running tubes quite hard on a regular basis also means replacing tubes on a regular basis.
