maybe it is your crossover freq effecting the total bass output when mains are set to small.
what is crossover set at the reciever ?
what is crossover set at the subwoofer ?

another thing to consider is the room harmonics. Have you played around with the polarity switch on the sub, or with room placement of the sub ?

hth

Randy

Axiom M80, Ep600, Qs8, VP150, Crown XLI 2500 , DBX Driverack PA2, Focusrite Scarlet 2i4