The following factors, which have not really been discussed in this thread, are highly relevant to any comparison of tube amps, namely, the amp's weight, the amp's price and the listener's choice of tubes.
As for tubes and output tubes in particular, their contribution to the performance and character of a tube amp is monumental -- people shouldn't say anything about the sonic character of a given tube amp unless they have spent a few hundred hours each with three or four different sets of output tubes on that amp.
If you really want to judge a tube amp in a general sense, however, the best way, in my experience, is to look at its weight and price. Much of the cost of a tube amp is determined by the cost of its output transformers and power supplies, the better output transformers and power supplies both being progressively heavier and more expensive. Although rated at only 100 watts per channel, the discontinued CAT JL-1 Limited Edition monoblocks cost $50,000, weighed 192 lbs. each, were all point-to-point wired, and could drive speakers better than monstrous solid-state amps. Why? Because they had incredible power supply capacitance (more, for example, than the 600 watt per channel ARC Reference 600 monoblocks, which, it should be noted, weigh 22 lbs. less), had outrageous output transformers that weighed 55 lbs. each and cost a fortune, and used very specially culled output tubes.
Another indicia of quality is whether the amp uses circuit boards, which make assembly less time-consuming and thus cheaper, or discreet transistors which are individually soldered in and thus more time consuming and costly to use ("point-to-point wiring"). Due to both the high cost of point-to-point wiring and general contraction over the last ten years of the two-channel tube-gear market at the expense of easier-to-operate multi-channel solid-state gear (I did not write "easier to own" -- tube amps are ultimately easier to own in my opinion, as they tend to be easier to fix if they break and basically become "new" when they are re-tubed -- some older solid-state gear cannot be fixed at all because the transistors are no longer made), it is is now hard to find a tube amp that is point-to-point wired. Highly respected manufacturers like VAC and CAT used point-to-point in the 90's, but have stopped doing it. Atma-Sphere still uses point-to-point exclusively, which is one reason why Atma-Sphere gear is expensive.
In any event, if you need a friend to help you lift it and it cost more than your Toyota, you can be pretty sure that you've got a good tube amp.
