The voltage listed on the transformer is the RMS voltage which can also be looked at as the DC equivalent in terms of power. The peak voltage is sqrt2 more than that (1.414x).
So a 25V transformer will output 35V peak which is the theoretical voltage you'll get after rectification and smoothing. In practice you'll loose a few volts in the rectifier diodes.
The maximum voltage swing you'll get out of the amplifier will be a bit less than that again due to losses in the output transistors. So for a 25V transformer and amplifier output stage you're looking at a practical limit of 28-30V peak output. Into an 8R load that gives a maximum output of about 56WRMS.
