I did some measurements on the sub and the calibration signal. I noticed that the sub cal signal is wide band so it is assuming a band limit on the sub itself to set the bandwidth and thus the power level that any meter will read. I think they want you to set the sub to 70Hz before calibration and then move it back after if you need it wider band. The 990's crossover works well on the sub, but not during calibration.

I calibrated with a noise source that I can adjust the bandwidth and frequency on. I got good results at a setting much below what I got calibrating with the internal source either with the Radio Shack meter or the internal calibration.