Tubes are also called "valves" for the reason they act like water valves. Think of the power supply as a water reservoir and the speakers water turbines. The more water that hits the turbine, the more the sound power. If you have only one valve, then the water flow to the turbine is limited to the valve let-thru. If you use two valves, you double the water flow, and the power. Three valves, four...the more water and the more power. But the ultimate limit to the water flow is the reservoir (power supply transformer and caps).
The number of output devices is picked to correspond to the power (current) desired and the power supply has to correspond to the current draw of these devices so the power is maintained. The quality of the sound depends on many many other factors. But as a rule of thumb, the more output devices means the more careful (read: expensive)the circuit design, since there are more devices it's easier to corrupt the current flow.
The number of output devices is picked to correspond to the power (current) desired and the power supply has to correspond to the current draw of these devices so the power is maintained. The quality of the sound depends on many many other factors. But as a rule of thumb, the more output devices means the more careful (read: expensive)the circuit design, since there are more devices it's easier to corrupt the current flow.