Dale,
150 watts is quite powerful for most studio arrangements. Your monitors in of themselves produce more than enough low end, the sub is there to accent and give a "big picture" effect. A sub woofer in a studio setup is different than the sub woofer in a home theater or stereo setup.
Monitors, inclusive of the sub woofer, in a studio application are for reference. Meaning they serve to relay to the engineer or artist an accurate and...