Hi,
If you need variable voltage, I would connect an analog output (+-10V) to a programming input of a powersupply, e.g. some Kepco types provide this. I'm using a RKW 12 -27K and some BOP models, which I control with an analog output channels of daqcards.
Check this out:
http://www.kepcopower.com/ecom.htm
Or you control your heaters using fix voltage power supplies which you switch through digital output with adequate optocouplers and power relais connected (cheaper solution)
Ciao,
Olli