Class A bias, speaker sensitivity, watts...?

Most solid state amplifiers can double the output power when impedance is halved if well under the max power spec. Not one amplifier I have seen measured by a third party has the ability to double max power when impedance is halved (not even the big Krell amps) the 8ohm spec is usually underrated to give this illusion. 

@invalid

True.  Even the electron has its spin!

Would like to try a really accomplished class A someday but don't want to deal with heat.  Same with tubes I suppose.  Purportedly, some class D can sound very class A like and that could be the best option.

But if space suffers, instrument sep. and stage depth isn't there, I wouldn't like it overall regardless if how smooth it sounded.

If you have 10 watts of class A, it should perform well at both low to moderate SPLs including dynamic swings.  The occasion peaks into class AB, etc probably wouldn't be audible until you turn it up to louder levels and at that point, its hard to listen too for longer periods of time and without risk to hearing.

Re current capabilities, is there a measurement, or some type of spec number? I would imagine that the µ/m-Farad value of the caps would be an indicator, but then there is also the maximum discharge speed F/s (= slew rate???) playing a role.

Capacitor values are not frequently given in specs. Tried to find it for my Pass XA25; there was a bulletin board mention of 18x 10K µF caps = 180 mF. If correct, is that a lot or not? Not the foggiest. How is discharge rate affected by cap size? I would imagine that many small caps discharge faster than one large one (assuming same total F value).

@richardbrand

I found an explanation for, or at least support for, the idea that Class A watts halve as speaker impedance halves. It’s from the December 2023 Stereophile review of the Accuphase A-300 amp. Her’s the link and the relevant quote:

Accuphase A-300 monoblock power amplifier | Stereophile.com

I also wrote to Duffey, who relayed the question to Inokuma and forwarded his response. "Here are our thoughts and technical approach to the question. For a push-pull output stage using a bipolar transistor as the output device, it operates as class-A up to twice the idling current flowing to the output device when the output is zero. If more than this amount of current flows through the speaker, one of the output devices, operating as a +/– pair, will turn off. Of course, current can be supplied to the outputs without any problem, and this is called class-AB amplification.

"In the case of the A-300, the idling current is applied so that the class-A range into 8 ohms is 125W. The amplifier operates as class-A up to 62.5W into 4 ohms and 31.25W into 2 ohms."

This inverse relationship between class-A power and load impedance is easily understood when you consider that current—idle current—determines an amplifier’s class-A range and that power equals current squared times load impedance: Cut the impedance in half and the power is halved as well. "So, the A-300 does in fact provide 125W of class A power into 8 ohms," Duffey wrote. "Into 4 ohms, though, the amplifier’s fixed amount of ’idle current’ can support just 62.5W of class-A power." Inokuma’s response included a table relating impedance, rated power, class-A range, and maximum power at clipping rated at 1% THD. The table showed that the clipping power easily surpasses the rated maximum output power at each load impedance, reaching 1100W into 1 ohm. How much power does a person need?