The above comment about the Snapper gives me a good example to explain my point about output transformers and power supplies.
The Snapper puts out 100 watts per channel. The CAT JL-1 monoblock tube amps also put out 100 watts per channel. The Snapper, according to its website, has 180 joules of energy storage in its power supply. In contrast, the CAT has 1,000 joules of energy storage. The output transformer alone in each CAT monoblock weighs 55 lbs., the total weight of each monoblock being 192 lbs., with most of the weight being the transformer and power supply. The Snapper weighs 38 lbs., a mere toy by comparison. Top-shelf transformers and power supplies are not only extremely heavy, but are extremely expensive - the CAT amps, for example, retailed for $30,000 ten years ago (the Snapper retails for $4,000). With tube amps, cost and weight are directly correlated and tell you most of what you need to know about quality.
Most tube amps layer space well, tend to be organic-sounding, and have a nice midrange, but with all due respect, they do not, except for the very best ones, control woofers on speakers featuring low impedances in the bass - this is their Achilles heel.