The amplifier will still be rated at 25 watts per channel no matter if one or both pairs of speakers are connected.
The sound output level will correspond to 25 watts of power being consumed. If one pair is connected and playing at full power, the SPL is determined by the 25 watts. If two pairs are connected, the power consumed by each pair is 12.5 watts (assuming identical speakers) and the resultant SPL drop is 3dB per pair. But when you add the SPL of each of the speaker pairs, that raises the SPL by 3 db. Conservation of energy.
The sound output level will correspond to 25 watts of power being consumed. If one pair is connected and playing at full power, the SPL is determined by the 25 watts. If two pairs are connected, the power consumed by each pair is 12.5 watts (assuming identical speakers) and the resultant SPL drop is 3dB per pair. But when you add the SPL of each of the speaker pairs, that raises the SPL by 3 db. Conservation of energy.