Audio amplifier limiting output power
A 30-0-30 transformer would, when fed into a bridge rectifier and smoothing capacitor produce dc voltages of about +/-40 volts and not +/-30 volts. Here's a picture to help you: -



A 30V AC signal means\$^1\$ 30V RMS i.e. having the same heating effect as 30V DC but the AC waveform also has a peak and this peak (for a regular sine wave AC power voltage) is \$\sqrt2\$ higher than the RMS voltage number. A bridge rectifier will charge up the smoothing capacitor to the peak voltage (minus a couple of volts lost in the diodes) hence, 30V RMS becomes 42 volt peak which then becomes about 40 volt DC on the smoothing capacitors.

This is likely to overload the amplifier - it has a maximum supply of 84 volts and a few percent increase in your AC power voltage could cause it to fail or at least not work correctly.

But, assuming you created a dc power supply of +/- 30V then you could limit the peak power into your loadspeakers by adding a series resistance of a few ohm. You could also build a circuit that limited the voltage driving into the input of the amplifier i.e. a "clipper" circuit.



\$^1\$ without any other terms like peak-to-peak (p-p) or peak (pk), if the stated voltage for a signal is 30V the default meaning is RMS.