Power tubes are made in all different capacities- from flea power (1 watt or less to kilowatt ratings). How a tube is 'harnessed' dictates how well it sounds and the power it can manipulate. For example, a 6550 used in a single ended circuit can do 10-15 watts or a little more. A El84 can do about 5 watts in a single ended circuit. In push-pull, 6550's can produce 60 watts or so and the EL84 around 15-20 max. It all depends on the tube's implementation. Some circuits can draw more out of a tube than others, even with the same power supply and output transformers.
The term B+ is archaic and stems from when a radio's high voltage came from a battery. Today it's slang for the high voltage output in a tube amplifier's power supply.