I'll tell you what I think I know and someone can correct me. Class A watts approximately halve from 8 ohms into 4 ohms, so you have between 5 and 10 watts of class A power.
Accuphase does the calculation of watts being output at any given time, but they say the calculation is complex. Here is their explanation:
Indicates the True Power into the Speaker
The power into the speaker is the product of the amplifier’s output voltage (V) and the current (I); that is, W = V×I. While the voltage is easily measured with a voltmeter, the measurement of current is more complex. Typical power meters avoid this complexity by assuming that the load resistance is equivalent to the speaker's nominal impedance. For these meters, then, I = V/R, and therefore, W = V×V/R, and so W = V2 /R. But as Figure 1 clearly shows, the actual impedance of the speaker varies complexly according to the frequency. The load resistance changes considerably as the impedance moves from peaks to dips, causing the power to change as well. Because typical meters measure only the voltage, they are unable to calculate true power at any given time. The A-80 meter, in contrast. uses detection circuitry, installed at the outputs, to dynamically detect both the load voltage and the current. The meter converts these analog readings into digital values that are then used to calculate the power. It can therefore display the true power value even as the power fluctuates along with the impedance
They have a microprocessor in their amps do the calculation.
Just a guess, but I think that in a small office with 80 db peaks you would stay in class A watts most of the time and probably wouldn't notice if you went into class B watts.
