Mark, note that although the RS meter isn't of course perfectly accurate, the reason that it's "off" in the low bass is intentional. The C weighted curve is intended to approximately compensate for the Fletcher-Munson effect which makes bass sounds at the same level seem quieter than midrange sounds. So, the meter reads several dB lower than the actual level to reflect the response of the ear. Boosting the sub level to compensate would result in a response in the bass which would be closer to being audibly flat, although it can't be adjusted perfectly, of course.
