How about varying the voltage the DC2DC converter gets? Their rated output is based on their input voltage. It would generally be easier to regulate the 6 volts supply via a linear regulator to control the output. Alternately you could just use a bleeder resistor on the 150volt line to it's ground such as a POT. You need to provide more specs like current requirements and your exact reasons for trying to regulate the voltage before any real suggestions can be made.