The Wazoo is not a powerhouse but, within its capabilities, an excellent amp. What is the efficiency of the SFs? How loud do you need it?
Remember, the Wazoo is really a power amp with a potentiometer in front. There is no gain as with a separate preamp. Therefore, turning up the volume control is merely reducing the attenuation and, until there's audible overload, don't worry about the position of the knob.
OTOH, if you are still dissatisfied with the output, consider something with more than the Wazoo's 50wpc (IIRC).