P = V2/R
In the case of AC, V is the RMS voltage, but with an amplifier, it's easier to use the peak voltage, since that's dependant on the power supply. For a sinewave, the RMS power is half the peak power, therefore:
P = VP2/(2R)
Unfortunately, no amplifier is perfect. The maximum output voltage is always slightly less than the input voltage, so the actual maximum output power will be slightly less than the value calculated by the above formula. If the amplifier's datasheet says it can output more power than that calculated, using the above formula, then it's either lying or the output waveform is no longer sinusoidal. In short your amplifier can't really drive an 8 Ohm speaker with 25W and a nice sine wave, when operating off 20V, hence why the data sheet specifies a THD of 10%, which is horrible. The output will be clipping with a huge amount of distortion. The useful output power will be much less. You should look at the output power specification with a THD of 1% for guidance.
Another thing to note is the actual output power depends on the, input signal level, volume control setting and amplifier gain. You could use an amplifier capable of driving 100W into an 8 Ohm speaker, with a 10W speaker and it won't blow up, as long as you don't play it too loud.