What power is your amplifier rated at?
I'm guessing 100W.
I'd use some 100W light bulbs as loads. Several reasons, the exact resistance is not important, and they can definitely dissipate 100W, and they're cheap.
Set up a bulb with a series ammeter and a parallel voltmeter so you can measure both current and voltage. Connect the scope across the power supply.
Either change the bulbs (25W, 50W, 100W, ...) or connect several bulbs in parallel. If you've got 110V mains, expect each 100W bulb to dissipate around 40W at the lower voltage (it's not a trivial calculation because the resistance of the filament is non-linear).
I'm not an absolute specialist at determining the load you should use to simulate a full power test on the amplifier's power supply. There are many variables and the level is strongly influenced by the design shortcuts which may have been made. Following I have listed some of these issues. Proceed with care. If there are fuses in the secondary, that may help indicate a safe load (it will be some fraction of their value -- certainly no more than half of them)
Because you're providing a constant load, you do not want to exceed half the amplifier's rated power on each rail. Amplifiers generally have power supplies capable of less power than you might think. This is based on the general assumption that no music is a constant tone at one level, it is highly dynamic with large peaks and a much lower average. The capacitors (and they're generally large) provide the power for those peaks. Because of these peaks can be quite large, the peak output of the amplifier may well exceed the rated power (it may be 175W for a 100W amplifier) but it may be limited to (say) 4 cycles at 1kHz (1/250th of a second). Another thing is that due to various losses, for 50W of energy output to the speakers, somewhat more than 50W of power from the power supply is required. For all of those reasons, any full load testing of the power supply should be kept short (several seconds) and only attempted if there are no signs of distress at lower loads.