In theory, using square law devices like MOSFETs, you can more than double the output current into half the load. You very rarely see it done because some sort of degeneration is applied to them to obtain stability.
Practically no class AB amps are significantly biased into class A. If they are the manufacturer makes a real big deal about it. The typical bias for a class AB amp is a very small fraction of a watt, not 5 or 10 watts.
Figuring out the power demands for a sound pressure at a distance are pretty straightforward, but that doesn't account for a speaker being a reactive load. To truly get a sense of a speaker's power demands you need a sense of the characteristics George describes above. Most amps are voltage sources and most speakers are designed to be driven by a voltage source. In other words, they're built to respond flatly to voltage, not the current or the watt. It's kind of assumed that the amp will deliver whatever current the load demands for the voltage. Higher impedance, less reactive speakers do better with current sources though.
All that said, there's nothing inherently superior about an amp that can double it's output. There's plenty of great amps that halve their output into half the load. My F5 delivers 52 watts into 8 ohm inside the class A envelope, but that envelope collapses to 18 watts into 2.8 ohm, the low point of my Focals. Yeah, it can and probably does drive way more power into that low point in class AB, but it's still an excellent sounding amp. My point is just looking at one number like it's power output ability doesn't tell much as one might think.
Practically no class AB amps are significantly biased into class A. If they are the manufacturer makes a real big deal about it. The typical bias for a class AB amp is a very small fraction of a watt, not 5 or 10 watts.
Figuring out the power demands for a sound pressure at a distance are pretty straightforward, but that doesn't account for a speaker being a reactive load. To truly get a sense of a speaker's power demands you need a sense of the characteristics George describes above. Most amps are voltage sources and most speakers are designed to be driven by a voltage source. In other words, they're built to respond flatly to voltage, not the current or the watt. It's kind of assumed that the amp will deliver whatever current the load demands for the voltage. Higher impedance, less reactive speakers do better with current sources though.
All that said, there's nothing inherently superior about an amp that can double it's output. There's plenty of great amps that halve their output into half the load. My F5 delivers 52 watts into 8 ohm inside the class A envelope, but that envelope collapses to 18 watts into 2.8 ohm, the low point of my Focals. Yeah, it can and probably does drive way more power into that low point in class AB, but it's still an excellent sounding amp. My point is just looking at one number like it's power output ability doesn't tell much as one might think.