Bob, as I said above, I had QS8s in the rear that never did anything for me. Today I put up 2 x M3v4 in the rear. I can't say they made an astonishing difference but they definitely filled in the back. I'd bet if I had my QS10s just a bit further back, the M3s wouldn't make as big a difference as they do now. Given your sides are more forward, you might want to consider M3s or something similar in the back.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated