Do you have a powered or passive subwoofer? Make/model?
If it's a powered sub, then have you set the sub's crossover to maximum?
If it's a passive sub and you have it attached to a speaker output on the receiver (not the sub out), then you will not hear the low frequencies since the LFE channel is only sent to the subwoofer audio out.
I just bought Sony str hd520 receiver and have it hooked to to large front speakers, a center speaker, and a sub woofer, but very little bass even with bass turned all the way up in settings menu. I feel like there is a setting I am missing. Help?