I have backed off from this because it looks like a big project that I don't have enough time to devote to. Perhaps others can help, but anyone who does needs a lot more info:
You gave the current requirements in Watts. Watts is a power measurement. Current is measured in Amps. Can you point to a datasheet of the device(s) that you need to control? "Powerhead" is noninformative.
I'm guessing the current requirements would make this a candidate for pulse width modulation (PWM), but we won't know until we know what sort of device you are controlling (motor, solenoid, etc.).
A microcontroller is probably the ideal part to use as your controller. Do you have experience with them?
If you don't use a microcontroller, it could concievably be controlled with an oscillator, a counter, some logic gates, a D/A converter, and either a PWM circuit or a linear driver, again depending on the load.
You need to post detailed ideas on how you want the control voltage vs time to look.
God knows what other questions will come up.