The MAIN volume on the receiver may be at -28dB, but what about the volume on the back of the sub? Also, what is the dB level of the Sub channel on the receiver? Also, how do you have your speakers setup in the receiver, small or large, and what crossover setting?
Here is my settings:
Sub - Volume/Gain knob about halfway between Min. and 9o'clock position. Crossover knob set to bypass. Trim set to Flat.
Receiver - All speakers set to small, crossover 80hz, dB level of sub channel about -4dB.
My guess is some of your settings are wrong.