Hollow, weak, with an Onkyo 806, sounds like something is wrong. I wouldn't pay attention to the -81 - 55 db read out as it is just a guide. I listen to my system on my Denon around -30. Have you used an SPL meter to set everything up or the on board set up routine? I would try resetting the Onkyo to factory specs and recalibrate with just an SPL meter to see how that sounds.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70