As I wrote in my earlier post about the amplifier power, the power is the product of two things: the voltage and the current. The higher the loudspeaker impedance (typically 6 Ohm or more), it is rather the voltage that sets the power limit. Consequently, the lower the loudspeaker impedance (4 Ohm or less), the chances are the amplifier will first reach its current limit.
The Model A was since its release offered in 25 W and 50 W versions, the only difference between them being the supply voltage. A 50 W version also had a transformer dimensioned adequately to follow the current demands. In any case, the Model A was able to provide 5 A, and such capabilities were enough to respond to typical loudspeaker demands for current.
Still, even though it did not happen very often, some Model A users also had speakers with an impedance that hit below 4 Ohm. In such circumstances, and considering the regular home listening environment and usual loudness demands, more current was necessary. That is why the Model A units which were shipped in 2015, as opposed to earlier devices, can provide 10 A. Therefore, at 4 Ohm these units double their claimed power at 8 Ohm, and can also work well with loudspeakers impedance as low as 3.5 Ohm.
It is also important to say that this change, while it improves the Model A capability, is done in a way that does not compromise its sound quality.