More thoughts...what crossover setting have you selected for the main speakers? It sounds like you may have it set low.

You can try setting it higher, say 100 Hz and see if that changes anything.

Though I see now that you can't move the sub/table to a corner, try moving the SPL meter around the room and note any changes. You might at least be able to be sure the sub isn't centered in the room.

You can also try temporarily connecting each VC of the Shiva seperately to 2 channels of the 7100 (Y-jack from the 950's SW out), which will boost your amp power to 2oo watts total.

I agree that you should be hearing more output than you describe.

Make sure the cable from the 950 to the 7100 is good.