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

@tomcy6

I've been trying to think of the physics that may be responsible.  When an AB amplifier is in Class A. the bias voltage is sufficient to accommodate the signal swings.  If the impedance of the speaker drops, more current is needed to keep the output voltage at the right level, but the current also tends to make the voltage drop.  I'll surmise that if the amplifier cannot keep up with the current demand, the bias voltage could drop sufficiently to actually reduce the Class A power.

According to Stereophile, the OP's Vincent Audio SV236MK measured 150-Watts into 8-Ohms and 231 in 4-Ohms.  It is specified at 10-Watts Class A into 8-Ohms but not specified into 4-Ohms.  Pity!

@richardbrand

There is only one number given for Class A watts that I’ve seen, which is why it stuck with me when the guy said they dropped into 4 ohms. It’s nothing to spend your time on, If someone who knows for sure comes across this thread maybe we’ll find out, but until then, forget I mentioned it.

@tomcy6 

What was it you mentioned?

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!