With due respect to many of the contributors to this thread, a lot of the information here reflects a misunderstanding of tube amps and what is important in a tube amp.
The ability of a tube amp to properly drive speakers is not so much determined by its wattage rating, but rather, by the quality of its output transformers (which convert voltage to current, current being needed to drive speakers featuring low impedences in the bass frequencies) and the size of its power supplies. It is generally easy to determine whether a given tube amp has quality transformers and power supplies: if the amp is heavy as hell and expensive as hell, you generally have a fine tube amp, as high-quality output transformers are both extremely heavy and costly (power supplies are likewise heavier the bigger they are). To make a rough analogy, a high-powered tube amp with mediocre output transformers is like a high-horsepower engine mated to a bad transmission -- if you can't get the power to the wheels and to the road, it doesn't matter how many horses you have. There are some very well known, supposedly "high-powered" tube amps out there that cannot drive tough loads because mediocre output transformers were used.
Generally speaking, it is very hard to find a tube amp that retails for less than say, $15k, that will properly drive dynamic-driver, low-impedence speakers, and $20k+ is more typical. In my experience, the CAT amps are by far the best in this regard, my VAC Renaissance amps being somewhat less good but still very good.
In my opinion, unless you are willing to commit real money to tube amplification, you're usually better off running a good solid-state amp.
