The first watts

Sound pressure falls off by the square of the distance. By the time you are 9 feet back this can be significant.

2 meters is thus 1/4 the energy. The speakers, once the conversion is done from sensitivity to efficiency, are 84 db 1 watt/1 meter (87 db 2.83V/1 meter and a 4 ohm speaker).

so 1 watt at 2 meters is 78 db. We're a little further back than that of course. 10 watts is 88 db (10x more power). Add 3 db for the other channel. Discount for absorptive materials in the room- carpet, sofa, etc, as well as being further back.

You may assume that the amplifier is making significant class AB power- the 10 watts is likely getting used up pretty quick unless you have a very lively room. However that first 10 watts is likely helping out with low lever detail.

Note...I wouldn't be too concerned about this...a friend has a switchable A to A/B amp and the differences were nominal...although "golden ear" types will push pure class A...the irony...the majority of normal listening requires very little power...and operates in class A anyways...

^^ For the most part, I would not either, whether the amp is tube or solid state. Due to the rather low efficiency of the speakers, power is likely going to be more important than what class of operation is employed.

Atmasphere nailed it...