The limiting factor IS wattage. Tube amps have a high output impedance and low current which means their power is limited to the output tube plate dissipation. A transformer's needed to reflect the low speaker impedance and this limits the amplifier's power output. When impedance is high, the current is lower and within the amp's power rating (but will put out less power), as the impedance drops the amp clips. Softly, it's agreed.
A solid state amp has a low output impedance and higher current which in turn requires higher capacitance and, more important, a power transformer rated higher than the amp's output power. So SS amps usually have more in reserve and put out more juice as speaker impedance drops. Maybe a 20 wpc amp at 8 ohms can put out 30 or 40 wpc at 4 ohms. Any way you slice it, this is twice the power at that given load. And the load is the defining part. You cannot say they're the same or that a tube amp is more powerful unless you consider the same range of impedances - an area where tubes generally cannot compete.
BTW 20 watt Class A amps consume more power than 20 watt Class B amps if both are rated the same - they are not more powerful watt for watt. They're just power pigs; a 20 watt Class A amp requires a 100 watt minimum power supply whereas a Class B may require a 70 watt supply.
A solid state amp has a low output impedance and higher current which in turn requires higher capacitance and, more important, a power transformer rated higher than the amp's output power. So SS amps usually have more in reserve and put out more juice as speaker impedance drops. Maybe a 20 wpc amp at 8 ohms can put out 30 or 40 wpc at 4 ohms. Any way you slice it, this is twice the power at that given load. And the load is the defining part. You cannot say they're the same or that a tube amp is more powerful unless you consider the same range of impedances - an area where tubes generally cannot compete.
BTW 20 watt Class A amps consume more power than 20 watt Class B amps if both are rated the same - they are not more powerful watt for watt. They're just power pigs; a 20 watt Class A amp requires a 100 watt minimum power supply whereas a Class B may require a 70 watt supply.