+5.5 on the receiver + 3/4 gain on the sub is normally pretty high ... unless you have the master volume pretty low.
do you have an SPL meter? if not, you need to stop what you are doing right now and order one!
normally, I start my sub calibration with the subwoofer gain at 0db. then i adjust the subwoofer gain to match the other channels on the SPL meter.