Have you tried tweaking the other settings you have? I'm not sure about your HU but on my Pioneer there are a bunch of settings: Bass Boost, Sub Freq. Boost/Gain, Loudness setting, 3-Band EQ, Parametric EQ Adjustments, SLA, Sound Retreiver etc...
All these will ultimately come into play when determining how much power the HU is outputting to the subs... try dicking around with your settings and then set your gains accordingly.