Powering an AC induction motor with a function generator and an audio amp
You should apply 2 volts per Hz, 70 volts for 35 Hz. You shouldn't apply a higher voltage than 2 X Hz. You should apply power at a reduced frequency and then increase the frequency and voltage to accelerate the motor rather than just switching on the motor. You don't need a very good sine wave.

Since you are working with 40 W, 120 V, 60 Hz shaded pole motors, they are undoubtedly single-phase motors. Variable frequency operation should be possible. Since the loads are fans, the load torque diminishes drastically as speed is reduced. That helps with variable frequency operation.

You can probably buy a small variable frequency drive (VFD) for not much more than the price of your example power amplifiers. You can buy VFDs designed for 120 volt, single-phase input, but the output will be stepped up to 240 V, 3 phase. The output voltage can probably be programmed to 120 volts. That will cut the power rating in half, so you will need a 500 W, 3/4 Hp VFD for 2 motors per phase.