Right on the 3 dB.

I made a mistake. Stacking two 500s will result in 6dB of increase. So if you stack two 500s, you'll get 114 anechoically and 124 in room which is the same as an EP800.

But you won't get the deep bottom end of the 800 as you know.

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